Various security agencies have raided a flat in a Boston suburb. Officers from the FBI, the Department of Homeland Security and US Immigration and Customs Enforcement searched the fifth floor flat in Revere, five miles from the site of the blast.
They were then seen leaving the property with a brown paper bag.
Two unexploded devices recovered near the attack will be analysed today, including one reportedly found around the corner from the first blasts. A third device was reportedly destroyed in a controlled explosion.
The overnight death toll after the Boston marathon bombing had risen to three this morning as federal and local investigators swarmed the city to try to hunt down those responsible for a pair of explosions that also caused injuries to at least 140 other people, at least 17 of whom remain in a critical condition.
As news emerged that one of the dead was an eight-year-old child, doctors from area hospitals began to report that many of those admitted had suffered from shrapnel injuries. The devices had been packed with ball bearings, according to reports, to cause maximum injuries. Hospitals also confirmed that numerous patients suffered lower limb injuries including some who had undergone amputations.
"These runners just finished and they don't have legs now," Roupen Bastajian, 35, a Rhode Island state trooper and a former Marine was cited as saying in the New York Times. "So many of them. There are so many people without legs. It's all blood. There's blood everywhere. You got bones, fragments. It's disgusting." A runner in the race himself, he added: "We put tourniquets on. I tied at least five, six legs with tourniquets."
The horror delivered a cold shock to the entire country as evidence mounted that this was a terrorist attack even if it remained unclear if it might be traced to a domestic group or individual or to an international terror cell like al-Qa'ida. Some analysts noted that Monday marked the start of a week that includes Patriot Day an important date to anti-government groups. The US is also coming up on the 19th April anniversary of the attack on the federal offices in Oklahoma City.
There was a sense of curfew in Boston as the FBI and local police sealed streets and scrambled to investigate. "We don't yet have some of the answers," President Barack Obama said. The word "terror" did not leave his lips. "We still don't know who did this or why...but make no mistake we will get to the bottom of this and we will find who did this and we will find out why they did this," he said, adding that the guilty "will feel the full weight of justice".
"There is no suspect in custody, we are questioning many people but there is no one in custody at this moment," Commissioner Ed Davis said in Boston last night without giving further details on a death toll. He said only: "There are people who have expired as a result of this incident.
In the immediate aftermath of the marathon explosions a plume of smoke could be seen rising above the Charles river, while video from helicopters at the scene revealed pools of blood spilled over a large area of pavement on Boylston Street. But while the carnage was vivid and real, there was the realisation that it could have been worse. Officials said the bombs were relatively rudimentary in nature, not stacked with sophisticated explosives.
The attack triggered security precautions in other cities, including the closing down of Pennsylvania Avenue close to the White House in Washington to foot traffic. Counter-terrorism police were also deployed to major landmarks in Manhattan and strategic locations across the country. Sporting events since 9/11 have been of particular concern for counter-terrorism forces. Huge protection is routinely provided for events from the World Series baseball tournament to the Super Bowl as well as the annual New Year's Eve celebration in Times Square.
In Los Angeles, the Sheriff's Department activated its emergency operations centre and increased patrols at transit hubs, schools and county buildings, while in New York, critical response teams were deployed citywide and officials stepped up security at hotels and other prominent locations. California emergency management officials activated their statewide threat assessment system, established after the 9/11 attacks. And officials in multiple cities and counties throughout the state were reviewing information from federal authorities for possible threats.
The blasts came within seconds of each other some three hours after the winners of the race had already crossed the line, but stragglers were still approaching the area close to the heart of the city's downtown. Police and medical personnel rushed to the scene as terrified members of the public attempted to flee. Many of those injured were treated on the spot before being transferred to the area's hospitals, including Massachusetts General.
Television footage from the scene showed injured victims being treated on the pavement, many of them with severe injuries. The bombs went off in the city's popular Back Bay retail district, blowing out windows in the surrounding buildings. The area was particularly crowded as one of the prime viewing spots for the annual race.
"I heard two big explosions, large plumes of smoke, dust, glass ... at first it seemed like a cannon blast but it felt so strong it almost blew my head off. There was some really, really bad injuries, some people were really badly hurt," said one spectator who rushed to site of the explosions.
Cherie Falgoust was waiting for her husband, who was running the race. "I was expecting [him] 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."
In an all-out response to the horror, the city closed the T-line, a subway that runs beneath Boylston, and sealed the nearby Massachusetts Bridge. All off-duty Boston police officers were summoned and told to report for emergency duty to the local precinct headquarters. As soon as the wounded were attended to, work began on an investigation into the sources of the blasts with the early focus looking for forensic evidence and anything suspicious seen on surveillance cameras.
It was a public holiday in Massachusetts and the Boston marathon is one of the most prestigious sporting events in America's calendar and for the city it is usually a joyous day that marks the end of winter. There were more than 24,000 runners representing 96 countries taking part, while at least half a million people lined the route.
Vice-President Joe Biden was on a conference call with gun control activists when staffers turned on televisions in his office after hearing of the explosions.
"Apparently there has been a bombing," Mr Biden was heard to comment. "I don't know any of the details of what caused it, who did it. I don't think it exists yet. But our prayers are with those people in Boston who suffered injury."
The city was where two of the flights hijacked on 9/11 originated from and the metropolis, on the Atlantic seaboard of Massachusetts, is as sensitive as any in the United States about the danger of terrorism attacks. Yesterday's explosions occurred at around 2.50pm local time.
Police in London said they were reviewing security plans for Sunday's marathon, the next major international event of its type, because of the explosions that hit the race in Boston.
"We are deeply saddened and shocked by the news from Boston," Nick Bitel, London Marathon Chief Executive, said in a statement.
"Our immediate thoughts are with the people there and their families. It is a very sad day for athletics and for our friends and colleagues in marathon running. Our security plan is developed jointly with the Metropolitan Police and we were in contact with them as soon as we heard the news."
By David Usborne, Independent News Service