Two bombs blew up seconds apart Monday at the finish line of one of the world's most storied races, tearing off limbs and leaving the streets spattered with blood and strewn with broken glass. Three people were killed, including an 8-year-old boy, and more than 170 were wounded.
A doctor treating the wounded said one of the victims was maimed by what looked like ball bearings or BBs.
Federal investigators said no one had claimed responsibility for the bombings, which took place on one of the city's biggest civic holidays, Patriots Day. But the blasts raised the specter of another terrorist attack on U.S. soil.
President Barack Obama said the bombings were an act of terrorism, but investigators do not know if they were carried out by an international organization, domestic group or a "malevolent individual." He said, "the American people refuse to be terrorized."
On Capitol Hill, Defense Secretary Chuck Hagel called the bombings "a cruel act of terror" and said "a thorough investigation will have to determine whether it was planned or carried out by a terror group, foreign or domestic."
Across the U.S., from Washington to Los Angeles, police tightened security, monitoring landmarks, government buildings, transit hubs and sporting events.
"We will go to the ends of the Earth to identify the subject or subjects who are responsible for this despicable crime, and we will do everything we can to bring them to justice," said Richard DesLauriers, FBI agent in charge in Boston.
He said investigators had received "voluminous tips" and were interviewing witnesses and analyzing the crime scene.
Gov. Deval Patrick said that contrary to earlier reports, no unexploded bombs were found. He said the only explosives were the ones that went off.
FBI agents searched a home in the suburb of Revere overnight. Authorities gave no details. But investigators were seen leaving a building there early Tuesday carrying brown paper bags, plastic trash bags and a duffel bag.
At a news conference, police and federal agents repeatedly appealed for any video, audio and photos taken by marathon spectators, even images that people might not think are significant.
"There has to be hundreds, if not thousands, of photos and videos" that might help investigators, state police Col. Timothy Alben said.
Boston Police Commissioner Edward Davis said investigators also gathered a large number of surveillance tapes from businesses in the area and intend to go through the video frame by frame.
"This is probably one of the most photographed areas in the country yesterday," he said.
Investigators refused to give any specifics on the bombs and say, for example, where they might have been hidden or whether they were packed with shrapnel for maximum carnage, as is often the case in terror bombings overseas.
But Dr. Stephen Epstein of the emergency medicine department at Beth Israel Deaconess Medical Center said he saw an X-ray of one victim's leg that had "what appears to be small, uniform, round objects throughout it -- similar in the appearance to BBs."
The fiery explosions took place about 10 seconds and about 100 yards apart, knocking spectators and at least one runner off their feet, shattering windows and sending columns of smoke rising over the street.
Roupen Bastajian, a state trooper from Smithfield, R.I., had just finished the race when he heard the explosions.
"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 17 people were critically injured, police said. At least eight children were being treated at hospitals. In addition to losing limbs, victims suffered broken bones, shrapnel wounds and ruptured eardrums.
At Massachusetts General Hospital, Alasdair Conn, chief of emergency services, said: "This is something I've never seen in my 25 years here ... this amount of carnage in the civilian population. This is what we expect from war."
Eight-year-old Martin Richard was among the dead, said U.S. Rep. Stephen Lynch, a family friend. The boy's mother, Denise, and 6-year-old sister, Jane, were badly injured. His brother and father were also watching the race but were not hurt.
A candle burned on the stoop of the family's single-family home in the city's Dorchester section Tuesday, and the word "Peace" was written in chalk on the front walk.
Neighbor Betty Delorey said Martin loved to climb the neighborhood trees, and hop the fence outside his home.
Tim Davey of Richmond, Va., was with his wife, Lisa, and children near a medical tent that had been set up to care for fatigued runners when the injured began arriving. "They just started bringing people in with no limbs," he said.
"Most everybody was conscious," Lisa Davey said. "They were very dazed."
The Boston Marathon is one of the world's oldest and most prestigious races and about 23,000 runners participated. Most of them had crossed the finish line by the time the bombs exploded, but thousands more were still completing the course.
The attack may have been timed for maximum bloodshed: The four-hour mark is typically a crowded time near the finish line because of the slow-but-steady recreational runners completing the race and because of all the friends and relatives clustered around to cheer them on.
Davis, the police commissioner, said authorities had received "no specific intelligence that anything was going to happen" at the race. On Tuesday, he said that two security sweeps of the route had been conducted before the marathon.
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.
Richard Barrett, the former U.N. coordinator for an al-Qaida and Taliban monitoring team who has also worked for British intelligence, said the relatively small size of the devices in Boston and the timing of the blasts suggest a domestic attack rather than an al-Qaida-inspired one.
"This happened on Patriots Day -- it is also the day Americans are supposed to have their taxes in -- and Boston is quite a symbolic city," said Barrett, now senior director at the Qatar International Academy for Security Studies.
The Pakistani Taliban, which has threatened attacks in the United States because of its support for the Pakistani government, on Tuesday denied any role in the bombings.
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 of 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 trickling down his head. Another person was on the ground covered in blood and not moving.
"My ears are zinging. Their ears are zinging," Wall said. "It was so forceful. It knocked us to the ground."
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