BOSTON (AP) -- Police say at least three people, including an 8-year-old boy, were killed in two explosions near the finish line of the Boston Marathon.
The explosions Monday also injured more than 150 people, knocking spectators and at least one runner off their feet.
In a morning news conference Tuesday, Massachusetts Gov. Deval Patrick said no unexploded bombs were found. He says the only explosives were the ones that went off Monday.
Police commissioner Ed Davis said it is the most complex crime scene in history of the department.
Some of the victims lost arms and legs. Other injuries included broken bones, shrapnel wounds and ruptured eardrums.
Within hours of the explosions, law enforcement officials, including those with the Bureau of Alcohol, Tobacco, and Firearms, were questioning a potential person of interest at Brigham and Women's Hospital, where many of the injured were taken, sources told ABC News.
Late Monday, the FBI has served a search warrant on a home in Revere in suburban Boston. Some investigators were seen leaving the house early this morning, carrying brown paper bags, plastic trash bags and a duffel bag. The FBI, Immigration and Homeland Security, K-9, a bomb squad and officials with the Alcohol, Tobacco and Firearms department were involved
The bombs went off almost simultaneously near the race finish line on Boylston Street before 3 p.m. The area was crowded with runners and spectators, and thousands of runners were still completing the race at the time of the first explosion.
A senior U.S. intelligence official said two other explosive devices were found near the end of the 26.2-mile course.
At the White House, President Barack Obama vowed that those responsible will "feel the full weight of justice."
There was no word on the motive or who may have launched the attack, and police said no suspect was in custody. Authorities in Washington said there was no immediate claim of responsibility.
"They just started bringing people in with no limbs," said runner Tim Davey, of Richmond, Va. He said he and his wife, Lisa, tried to keep their children's eyes shielded from the gruesome scene inside a medical tent that had been set up to care for fatigued runners, but "they saw a lot."
"They just kept filling up with more and more casualties," Lisa Davey said. "Most everybody was conscious. They were very dazed."
The twin blasts at the race took place almost simultaneously and about 100 yards apart, tearing limbs off numerous people, knocking spectators and at least one runner off their feet, shattering windows and sending smoke rising over the street.
Some 23,000 runners took part in the race, one of the world's oldest and most prestigious marathons. One of Boston's biggest annual events, the race winds up near Copley Square, not far from the landmark Prudential Center and the Boston Public Library. It is held on Patriots Day, which commemorates the first battles of the American Revolution, at Concord and Lexington in 1775.
Boston Police Commissioner Edward Davis asked people to stay indoors or go back to their hotel rooms and avoid crowds as bomb squads methodically checked parcels and bags left along the race route. He said investigators didn't know whether the bombs were planted in mailboxes or trash cans.
He said authorities had received "no specific intelligence that anything was going to happen" at the race.
The Federal Aviation Administration barred low-flying aircraft from within 3.5 miles of the site.
Obama was briefed on the explosions by Homeland Security adviser Lisa Monaco. Obama also told Mayor Tom Menino and Gov. Deval Patrick that his administration would provide whatever support was needed, the White House said.
"We still don't know who did this or why," Obama said, adding, "Make no mistake: We will get to the bottom of this."
About four hours into the race and two hours after the men's winner crossed the line, there was a loud explosion on the north side of Boylston Street, just before the photo bridge that marks the finish line. Another explosion could be heard a few seconds later.
By that point, more than 17,000 of the runners had finished the race, but thousands of others were farther back along the course.
A senior U.S. intelligence official said the two other explosive devices found nearby were being dismantled. The official spoke on condition of anonymity because he was not authorized to discuss the findings publicly.
A woman who was a few feet from the second bomb, Brighid Wall, 35, of Duxbury, said that when it exploded, runners and spectators froze, unsure of what to do. Her husband threw their children to the ground, lay on top of them and another man lay on top of them and said, "Don't get up, don't get up."
After a minute or so without another explosion, Wall said, she and her family headed to a Starbucks and out the back door through an alley. Around them, the windows off the bars and restaurants were blown out.
She said she saw six to eight people bleeding profusely, including one man who was kneeling, dazed, with blood coming down his head. Another person was on the ground covered in blood and not moving.
"My ears are zinging. Their ears are zinging. It was so forceful. It knocked us to the ground."
Competitors and race volunteers were crying as they fled the chaos. Authorities went onto the course to carry away the injured while race stragglers were rerouted away from the smoking site.
Roupen Bastajian, a 35-year-old state trooper from Smithfield, R.I., had just finished the race when they put the heat blanket wrap on him and he heard the blasts.
"I started running toward the blast. And there were people all over the floor," he said. "We started grabbing tourniquets and started tying legs. A lot of people amputated. ... At least 25 to 30 people have at least one leg missing, or an ankle missing, or two legs missing."
Smoke rose from the blasts, fluttering through the national flags lining the route. Blood stained the pavement in the popular shopping and tourist area known as the Back Bay.
Cherie Falgoust was waiting for her husband, who was running the race.
"I was expecting my husband any minute," she said. "I don't know what this building is ... it just blew. Just a big bomb, a loud boom, and then glass everywhere. Something hit my head. I don't know what it was. I just ducked."
Runners who had not finished the race were diverted straight down Commonwealth Avenue and into a family meeting area, according to an emergency plan that had been in place.
The Boston Marathon honored the victims of the Newtown, Conn., shooting with a special mile marker in Monday's race.
Boston Athletic Association president Joanne Flaminio previously said there was "special significance" to the fact that the race is 26.2 miles long and 26 people died at Sandy Hook Elementary school.
Associated Press writers Jay Lindsay, Steve LeBlanc, Bridget Murphy and Meghan Barr in Boston and Lara Jakes and Eileen Sullivan in Washington contributed to this report.