Police Commissioner Ed Davis said that no further devices had been found in the Boylston Street area, where three people were killed and more than a hundred others injured in the twin explosions at the finish line of the Boston Marathon. Mr. Davis said that the police department had been receiving reports from various places about suspicious packages. “I am not prepared to say that we are at ease at this point of time,” he said.

Police Commissioner Ed Davis said that three people had died as a result of injuries suffered in the twin explosions at the finish line of the Boston Marathon.

Gov. Deval Patrick said that Boston would be open for business on Tuesday, but that there would be increased security. He also asked that “everyone be on a state of heightened vigilance.”

Gov. Deval Patrick of Massachusetts said the Federal Bureau of Investigation would lead the investigation into Monday’s explosions in Boston.

Authorities in Boston are scheduled to hold a news conference shortly to discuss Monday’s explosions at the Boston Marathon.

“The Boston Athletic Association extends its deepest sympathies to all those who were affected in any way by today’s events.

Today is a sad day for the City of Boston, for the running community, and for all those who were here to enjoy the 117th running of the Boston Marathon. What was intended to be a day of joy and celebration quickly became a day in which running a marathon was of little importance.

We can confirm that all of the remaining runners who were out on the course when the tragic events unfolded have been returned to a community meeting area.

At this time, runners bags in Boston which remain unclaimed may be picked up by runners presenting their bib number or proof of race participation at the Castle, at 101 Arlington Street, in Boston.

At this time, we are cooperating with the City of Boston, the Commonwealth of Massachusetts, and all federal law enforcement officials.

We would like to thank the countless people from around the world who have reached out to support us today.”

SWAT teams suited in body armor and carrying assault rifles set up tight security perimeters around Boston hospitals, where more than 100 people were being treated for injuries sustained in Monday’s bombing.

The Boston police said the heightened security was standard practice for such an investigation.

There were reports that investigators were questioning at least one person at a local hospital.

There were also some reports that Massachusetts General Hospital was put on alert because of a suspicious package found in a parking garage, but the hospital said nothing was found.

— Michael Schwirtz

Shortly after the explosion, the Federal Aviation Administration issued a no-fly zone for the Copley Square area in Boston, where the twin blasts killed at least two people and wounded more than 100 others, according to The Associated Press.

The Associated Press also reported increased security on outbound international flights at Logan International Airport.

Several areas of Boston were evacuated after explosions that killed two and injured dozens at the Boston Marathon on Monday, as authorities worked to respond to the attacks at the marathon’s finish line and to secure other areas of the city.

Marathon runners and spectators were quickly evacuated from the immediate area of the attack, and a subway station serving the area was closed by the Massachusetts Department of Transportation.

Gov. Deval Patrick of Massachusetts posted a message to Twitter urging people to avoid Copley Square and leave the area to first responders. He also advised “nonessential people” against gathering at hospitals so that the injured could seek treatment.

The Boston Police Department said in an update on Twitter that the area around the explosion would be closed “for the foreseeable future.”

The police in Brookline, a town that borders Boston, said in an update on Twitter that they had evacuated an area near Washington Square and Coolidge Corner and urged residents to avoid the area.

They also robo-called residents to warn them to avoid a local main street because of the possible threat of explosives, according to a report on Twitter.

At least two universities in Boston evacuated parts of their campuses. Tufts University briefly evacuated its medical center because of a “suspicious package that turned out not to be a threat,” according to The Fall River Herald News.

Jim Lokay, a reporter with WCVB news in Boston, also reported that Tufts Medical Center was briefly evacuated but that no threat had been found there.

Harvard University evacuated its prestigious Kennedy School, according to an update posted to Twitter by Ya’akov Katz, an Israeli journalist and Nieman fellow at Harvard.

— Liam Stack

Billy Baker, a Boston Globe reporter, traveled around the city on Monday afternoon and posted frequent updates to Twitter on the anxious mood he observed.

— Liam Stack

WCVB-TV News presented a breakdown of where the more than 100 people injured, including at least a dozen children, were being treated at nearly a half-dozen hospitals.

The House speaker, John A. Boehner, issued the following statement: “Words cannot begin to express our sorrow for the families who are grieving so suddenly right now. The House of Representatives offers its prayers to the victims and the city of Boston. We also give thanks for the professionals and Good Samaritans who prevented further loss of life. This is a terrible day for all Americans, but we will carry on in the American spirit, and come together with grace and strength.”

The Associated Press reports that cellphone companies say “service is operating in the Boston area, but with heavy traffic following the explosions at the Boston Marathon.”

Earlier, a law enforcement official, citing an intelligence briefing, told A.P. that cellphone service had been shut down in the Boston area “to prevent any potential remote detonations of explosives.” But officials with Verizon Wireless and Sprint Nextel told the agency there had been no such requests.

A Sprint spokeswoman, Crystal Davis, said, “Minus some mild call blocking on our Boston network due to increased traffic, our service is operating normally.”

The National Hockey League has postponed Monday night’s Boston Bruins game against the Ottawa Senators at TD Garden.

The Bruins’ president, Cam Neely, issued the following statement: “After consultation with city, state and N.H.L. officials, we collectively made the decision to postpone tonight’s game. Public safety personnel from the city and state are still gathering information regarding today’s events and it is vital they have all resources available for their investigation. The thoughts and prayers of everyone in the Bruins organization are with the city of Boston and all those affected by today’s tragedy.”

The Boston Symphony Orchestra canceled its concert on Monday night after the bombings, citing “security reasons.” The orchestra’s managing director, Mark Volpe, said in an interview that with streets around Symphony Hall blocked off and the Green line of the T shut down, it would be difficult for audience members to get to the auditorium. While there was no specific threat, Mr. Volpe said security officials did a sweep of the building as a precaution and found nothing. He said that as he made his way to Symphony Hall this afternoon, he saw some people “visibly upset” and others “in a daze.”

“It’s shocking,” he said.

Symphony Hall is less than a mile from the marathon finish line.

— Daniel J. Wakin

Monday’s marathon was dedicated in part to the victims of December’s mass shooting at Sandy Hook Elementary School in Newtown, Conn. At the start of the marathon, 26 minutes of silence were held to honor the 26 victims, and the 26th mile marker was emblazoned with the Newtown seal.

A team from Newtown was among the hundreds of participants in the marathon. Laura Nowacki, a pediatrician who The Hartford Courant said lost eight patients in the shooting and whose fourth-grade daughter survived it, was on the team.

The Newtown Strong charity, with which the runners were affiliated, posted a message on Facebook confirming that all its runners were safe.

— Michael Schwirtz

President Obama declared on Monday that the person or groups responsible for the deadly explosions at the Boston Marathon would be hunted down and brought to justice, but he cautioned, “We still do not know who did this or why.”

“We will get to the bottom of this,” Mr. Obama said in a brief statement from the White House shortly after 6 p.m. “We will find out who did this, and we will find out why they did this. Any responsible individuals, any responsible groups will feel the full weight of justice.”

The president did not refer to the attacks as acts of terrorism, and he cautioned people against “jumping to conclusions” based on incomplete information.

But a White House official, speaking on condition of anonymity afterward, said, “Any event with multiple explosive devices, as this appears to be, is clearly an act of terror and will be approached as an act of terror.”

“However,” the official added, “we don’t yet know who carried out this attack, and a thorough investigation will have to determine whether it was planned and carried out by a terrorist group, foreign or domestic.”

In the hours since the attacks, Mr. Obama has been briefed by the homeland security secretary, Janet Napolitano; the F.B.I. director, Robert Mueller; and the White House’s chief adviser on counterterrorism and homeland security, Lisa Monaco.

Mr. Obama paid tribute to Boston as a “tough and resilient town,” saying, “I’m supremely confident that Bostonians will pull together, take care of each other and move forward as one proud city, and as they do, the American people will be with them every single step of the way.”

“The American people will say a prayer for Boston tonight,” Mr. Obama said.

— Mark Landler

Video posted online by Reuters shows the aftermath of the explosion as seen from a helicopter above Boylston Street.

— Robert Mackey

The Boston police said that they were questioning multiple people but that no suspects were in custody. No one has claimed responsibility for the explosions.

Gov. Deval Patrick of Massachusetts said that 15 blocks around Copley Square, where the explosions took place, would continue to be closed.

The Boston Globe has posted two spreadsheets via Google to connect runners with places to stay if they are unable to leave Boston because of the bombings. There is also a hot line available for families of victims: (617) 635-4500 and a tip line for information about the incident (800) 494-TIPS

President Obama is expected to make a statement on the explosions in Boston at 6:10 p.m. The White House will stream live video of the statement when it begins in the player below.

— Robert Mackey

Massachusetts General Hospital is posting updates on its Web site.

At least six others wounded in the explosion were taken to Tufts New England Medical Center, which just reopened its emergency room after a bomb scare.

According to the Boston Marathon organizers, “The Family Meeting Area has been moved to Boston Common. Runners are being directed there to meet friends and family. City of Boston assets have been deployed to assist runners at the Common.”

Google has launched a ‘Person Finder’ to help people find friends and loved ones in the aftermath of the Boston Marathon Explosions.

The Boston Globe has created two Google Docs for runners who need places to stay tonight and residents with rooms to offer.

— Robert Mackey

The Boston Police Department’s public information bureau chief, Cheryl Fiandaca, reports that the incident at the J.F.K. Library appears to be related to a fire, not an explosion.

— Robert Mackey

The Boston Globe has posted dramatic video of emergency service workers and bystanders rushing to help victims in the immediate aftermath of the explosion near the finish line of the marathon on Boylston Street.

— Robert Mackey

Boston’s police commissioner confirmed at a news conference that there was an explosion at the John F. Kennedy Library, located several miles from Copley Square next to the University of Massachusetts Boston campus. He said that the police were investigating whether there was a possible connection to the explosions at the finish of the Boston Marathon. Library officials said that it was too early to speculate and that it appeared the explosion at the J.F.K. Library took place in the mechanical room.

The Boston police commissioner initially said that a fire or explosion at the John F. Kennedy Library was possibly “related” to explosions at the finish line of the Boston Marathon. The police confirmed that a suspicious package was found and destroyed by police at the Mandarin hotel.

Later, the police said the library incident was related to a fire.

Steve Silva, a Boston Globe Web producer, captured dramatic video of the explosion at the finish line from close range. (The video might load slowly.)

— Robert Mackey

Our colleagues on the Graphics desk have produced a map showing the location of the two blasts near the finish line on Boylston Street.

— Robert Mackey

NBC Television reports that a a third possible device was located near the public library in Copley Square and was removed.

Video posted on YouTube appears to show a second explosion near the finish line.

Near the start of the brief clip, smoke can be seen in the distance just before the sound of a loud explosion reaches the camera. A Google Map search suggests that the video was recorded just outside a Walgreen’s at 841 Boylston Street.

View Larger Map

— Robert Mackey

In the immediate aftermath of explosions at the Boston Marathon, the New York Police Department stepped up security at landmarks and hotels, said Paul J. Browne, the department’s chief spokesman.

The police sent critical response vehicles to prominent locations around the city as a precaution “until more about the explosions in Boston is learned,” Mr. Browne said.

The Port Authority police also increased security at its bridges, tunnels and other facilities around New York and New Jersey. “While there are no specific threats to PA assets,” said Joe Dunne, the chief security officer, “all PAPD officers are on heightened alert and will continue to monitor developments and coordinate with the F.B.I. Joint Terrorist Task Force.”

Such deployments are routinely taken by the department at times of heightened potential threat.

Authorities in Washington also heightened security following initial reports of the explosions, according to a spokeswoman for the Metropolitan Police.

In Washington, police cleared a stretch of Pennsylvania Avenue in front of the White House as a security precaution. A witness said the crowds were pushed back to Lafayette Park, across the street.

A spokesman for the Secret Service, Edwin Donovan, said, “Out of an abundance of caution, we have expanded our security perimeter at the White House complex. It is not unusual to expand or contract these security perimeters.”

— J. David Goodman and Mark Landler

Live audio of public scanners of Boston Fire Department.

The Boston Marathon organizers report on Facebook that there were “two bombs that exploded near the finish line.”

— Robert Mackey

Cheryl Fiandaca, the Boston Police Department’s public information bureau chief, reports on Twitter that at least two people have been killed and 23 wounded.

— Robert Mackey

“The president has been notified of the incident in Boston,” said a White House official. “His administration is in contact with state and local authorities. He directed his administration to provide whatever assistance is necessary in the investigation and response.”

“Shortly after being notified of the incident around 3 p.m. Eastern time, the president received a briefing from Homeland Security adviser Lisa Monaco and other members of his senior White House staff in the Oval Office,” said a White House official. “The president called Boston Mayor Tom Menino and Massachusetts Gov. Deval Patrick to express his concern for those who were injured and to make clear that his administration is ready to provide needed support as they respond to the incident.”

— Mark Landler

The NBC News affiliate WHDH-TV captured video of the large explosion at the Boston Marathon finish line this afternoon.

— Robert Mackey

A senior United States official said that the Boston Police Department did not have any advance warning or information about a threat of an attack at the Boston Marathon, according to our colleague Michael S. Schmidt.

David L. Ryan, a Boston Globe photographer, posted this image of an explosion at the finish line on Twitter.

— Robert Mackey

Sean Kelly is also reporting that some victims lost limbs.

The local CBS station had a live camera at the finish line of the marathon. Here’s the live video view now, showing an ambulance taking wounded.