BOSTON (AP) - Two bombs exploded in the crowded streets near the finish line of the Boston Marathon on Monday, killing at least three people and injuring more than 140 in a bloody scene of shattered glass and severed limbs that raised alarms that terrorists might have struck again in the US.
A White House official speaking on condition of anonymity because the investigation was still unfolding said the attack was being treated as an act of terrorism.
President Barack Obama, speaking from the White House late Monday, pointedly avoided using the words "terror" or "terrorism," saying officials "still do not know who did this or why."
However, a White House official later said the explosions at one of the world's oldest and most prestigious race was being treated as terrorism.
"We will find out who did this. We'll find out why they did this," Mr Obama said in his brief statement. "Any responsible individuals, any responsible groups, will feel the full weight of justice."
As many as two unexploded bombs were also found near the end of the 42-kilometre course as part of what appeared to be a well-coordinated attack, but they were safely disarmed, according to a senior US intelligence official, who also spoke on condition of anonymity because of the continuing investigation.
The fiery twin blasts took place almost simultaneously and about 100 metres apart, knocking spectators and at least one runner off their feet, shattering windows and sending dense plumes of smoke rising over the street and through the fluttering national flags lining the course.
When the second bomb went off, the spectators' cheers turned to screams. As sirens blared, emergency workers and National Guardsmen assigned to the race for crowd control began climbing over and tearing down temporary fences to get to the blast site.
A pool of blood formed, and huge shards were missing from window panes as high as three stories.
"They just started bringing people in with no limbs," said runner Tim Davey, of Virginia. 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."
Police said three people were killed. An 8-year-old boy was among the dead, according to a person who talked to a friend of the family and spoke on condition of anonymity.
Hospitals reported at least 144 injured, at least 17 of them critically. The injuries ranged from cuts and bruises to amputations. Many victims suffered lower leg injuries and shrapnel wounds. Some suffered ruptured eardrums.
Some 23,000 runners took part in the race, which attracts more than 500,000 spectators and winds up in the heart of central Boston, near the landmark Prudential Center and the Boston Public Library. It is held on Patriots Day, a Massachusetts state holiday which commemorates the first battles of the American Revolution 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 precisely where the bombs were planted or whether they were hidden in mailboxes or trash cans.
He said authorities had received "no specific intelligence that anything was going to happen" at the race.
Mr Obama was briefed on the explosions by Homeland Security adviser Lisa Monaco. Mr Obama also told Boston Mayor Tom Menino and Gov. Deval Patrick that his administration would provide whatever support was needed, the White House said.
The Federal Aviation Administration created a no-fly zone over the site of the explosions, and briefly ordered flights bound for Boston's Logan International Airport held on the ground at airports around the US.
A few kilometres away from the finish line and around the same time, a fire broke out at the John F. Kennedy presidential library. The police commissioner said it may have been caused by an incendiary device but didn't appear to be related to the bombings.
The first loud explosion occurred on the north side of Boylston Street, just before the photo bridge that marks the finish line. The second explosion could be heard a few seconds later.
They occurred about four hours into the race and two hours after the men's winner crossed the line. By that point, more than 17,000 of the runners had finished the race, but thousands of others were farther back along the course.
The four-hour mark is typically a highly crowded time near the finish line - both because of the slow-but-steady recreational runners likely to be completing the race and because of all the relatives and friends clustered around to cheer them on.
Runners in the medical tent for treatment of dehydration or other race-related ills were pushed out to make room for victims of the bombing.
A senior US 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.
At the White House, the Secret Service expanded its security perimeter after the attacks, shutting down Pennsylvania Avenue and cordoning off the area with yellow police tape. Several Secret Service patrol cars blocked off entry points, although the White House was not on lockdown and tourists and other onlookers were still allowed in the park across the street.
At Congress, members of intelligence committees said they expected to be briefed on the attack on Tuesday.
A woman who was near the second bomb, Brighid Wall, 35, 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."
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 police officer from the neighboring state of Rhode Island, 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."
At Massachusetts General Hospital, said Alisdair Conn, chief of emergency services: "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." The Boston Marathon honored the victims of the December shooting in Newtown, Connecticut, 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.
Cities worldwide stepped up security following the explosions.
In Britain, police said they were reviewing security plans for Sunday's London Marathon, the next major international marathon. Thousands of people compete in the London Marathon every year, thronging the city's streets. London is also considered a top target for international terrorists.
A London Metropolitan Police spokesman confirmed Monday that police are working with marathon officials to review security plans for Sunday's event. The London race's chief executive, Nick Bitel, expressed shock and sadness about the situation in Boston, saying "it is a very sad day for athletics and for our friends in marathon running." In New York City, police spokesman Paul Browne said that critical response teams are deployed around the city. Officials were stepping up security at hotels and other prominent locations.
Spectator 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."