Boston police said at least three people are dead and at least 144 people are injured, including eight children, in a pair of bombings at the finish line of the Boston Marathon.

More than 23,000 people were taking part in Monday's marathon when two explosions detonated on the north side of Boylston Street. The explosions detonated about four hours after the start of the men's race, at 2:50 p.m., which meant thousands of runners were still on the course at the time.

Boston Police Commissioner Ed Davis said the explosions happened within seconds of each other, at two locations that were 50 to 100 yards apart, and that each explosion resulted in "multiple casualties."

Among the three dead is an 8-year-old boy, officials say. No details about the boy's identity have been released, but a friend of the family who spoke on condition of anonymity said the boy's mother and sister were also injured as they waited for the father to finish the race.

Officials said that at least 17 of those injured are in critical condition and at least 25 of them are in serious condition.

The victims are being treated at nine hospitals around Boston. The injuries they are being treated for range from cuts and bruises to amputations. Officials said at least 10 amputations have been performed.

Many victims suffered lower leg injuries and shrapnel wounds, which officials say indicates the device was low to the ground. Some suffered ruptured eardrums.

Mass General Trauma Surgeon Peter Fagenholz described the injuries as combined injuries, meaning the bone, soft tissue and vascular systems are injured.

He said medical staff members are pulling small bits of metal out of people in the emergency room, which analysts say suggests the bombs were designed to propel shrapnel.

Fagenholz said many of the victims will need to undergo repeat surgical procedures on Tuesday.

"It's just depressing," he said. "We take care of accidents all the time, but it's just depressing that this is intentional."

No suspect in custody, police say

Reports have abounded on social media about a possible suspect, but FBI officials have said they do not have any suspects in custody. 

Officials said they are questioning "persons of interest" but that no one has been identified as a suspect. Davis stressed that there was no suspect at Brigham and Women's Hospital.

"There is no suspect at Brigham & Women's Hospital as has been widely reported," he said. "There are people we are talking to."

The FBI is taking over the investigation, and they are conducting it jointly with the assistance of state and local officials. They said the investigation is active and ongoing, and declined to release details during a press conference Friday evening.

Earlier in the day, a senior U.S. intelligence official says two more explosive devices were found near the scene of the Boston marathon where the two bombs detonated. The official said the new devices were being dismantled. 

There has been no further update on those two devices.

Davis also said a third explosion had occurred at the John F. Kennedy Presidential Library and Museum at about 4:30 p.m.

Later, authorities said the fire was not related to the two bombings. No one was injured in the fire. According to the JFK Library's Twitter feed, the fire appeared to have started in the mechanical room of their new building.

President: Suspect will feel 'full weight of justice'

President Barack Obama spoke about the marathon bombings later on Monday. He said that investigators do not have "all the answers" in the bombings.

"We still do not know who did this or why, and people shouldn't jump to conclusions before we have the facts," he said. "But make no mistake: we will get to the bottom of this. We will find out who did this. Any responsible individuals, any responsible groups will feel the full weight of justice."

Obama expressed confidence in the people of Boston in the aftermath of the bombings.

"Boston is a tough and resilient town," he said. "So are its people. I am supremely confident that Bostonians will pull together, take care of each other, and move forward as one proud city and as they do, Americans will be with them every single step of the way."

Heightened security around Boston

Davis has recommended that people either stay home or return to their hotels, and that they do not congregate in large crowds. Any bags that were left behind by runners and participants are being treated as suspicious packages.

Runners who were unable to finish the race are being brought to a support center, where they will have the opportunity to connect with family members and staff members who can help meet any needs they might have.

Two telephone numbers are being set up. Families of victims who are trying to locate people should call 617-635-4500. Anyone who says they saw anything that might be relevant to the investigation should call 1-800-494-TIPS or 1-800-CALL-FBI.

The FAA has restricted airspace over Boylston Street in Boston. The notice said the restriction was effective immediately and would be in place until further notice.

The agency said in a notice issued Monday about an hour after the explosions that a no-fly zone with a 3.5-mile radius has been created over 811 Boylston Street. The zone is limited to flights under 3,000 feet in altitude, which is lower than most airliners would fly except when taking off or landing.

The bridges over the Charles River have also been shut down as a precaution. The U.S. Coast Guard is stepping up patrols in Boston Harbor as well. 

Gov. Deval Patrick said heightened security would be visible around Boston, including on the T. 

An earlier report had indicated that cellphone service in the Boston area was shut down, but at about 5:30 p.m., cell phone companies said service is operating, but with heavy traffic following of the explosions at the Boston Marathon.

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

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

Spectators describe scene of explosions

Bloody spectators were being carried Monday to the medical tent that had been set up to care for fatigued runners. Police wove through competitors as they ran back toward the course.

"There are a lot of people down," said one man, whose bib No. 17528 identified him as Frank Deruyter of North Carolina. He was not injured, but marathon workers were carrying one woman, who did not appear to be a runner, to the medical area as blood gushed from her leg.

A Boston police officer was wheeled from the course with a leg injury that was bleeding.

One woman says she was waiting for her husband to cross the finish line, and, in her words, "it just blew." She described it as "a loud boom, and then glass everywhere."

Cherie Falgoust says something hit her head, and she "just ducked."

A runner, Laura McLean of Toronto, says she heard two explosions outside the medical tent. She says, "There are people who are really, really bloody."

McLean says, "They were pulling them into the medical tent."

Security beefed up worldwide in wake of explosions

Police in Los Angeles, New York City, London and other cities worldwide are stepping up security following explosions at the Boston Marathon.

Los Angeles County Sheriff's spokesman Steve Whitmore says the department has opened an emergency operations center, increased patrols for transit and other critical areas including the Los Angeles Dodgers game Monday night.

Chief NYPD spokesman Paul Browne said Monday that critical response teams are deployed around the city. Officials are stepping up security at hotels and other prominent locations.

British police also say they are reviewing security plans for Sunday's London Marathon. It's the next major international marathon. A London Metropolitan Police spokesman says police are working with marathon officials to review security plans.

Officials have also tightened security at the state's only nuclear power plant. The Pilgrim Nuclear Power Station in Plymouth, Mass., about 45 miles south of Boston, was already shut down for refueling when the explosions occurred.

Information from the Associated Press was used in this article.