Boston: US investigators stepped up a massive manhunt Tuesday for attackers who devastated the Boston Marathon with two bombs that killed three people and wounded more than 140, many left with horrific injuries.
Boston's Boylston Street, scene of the marathon finish line carnage on Monday, remained sealed off as investigators sought leads in the worst bomb attack in the United States since the September 11, 2001 atrocities.
President Barack Obama vowed that the attackers "will feel the full weight of justice", police searched the apartment of one "person of interest" and a Saudi man remained under guard in hospital. But there were no convincing answers for a city in mourning.
According to the latest toll, the two bombs, which were 13 seconds and about 100 meters (yards) apart, left 17 people in critical condition.
The dead and injured were aged between two and 71, hospital authorities said. Nine children were injured.
Among the dead was an eight-year-old local boy, Martin Richard, who had been waiting at the finish for his father to cross the line. His mother and sister, who lost a leg, also suffered critical injuries.
Many of the wounded lost limbs in the blasts, which sent metal shrapnel flying into the crowd of thousands of people packing Boylston Street, traditionally a venue of joyful partying at the end of the world's oldest international marathon.
Liz Norden tearfully told the Boston Globe how two of her sons, both in their 30s, lost legs in the blast.
Most of the 23,000 runners participating in the 26.2 mile (42 kilometer) race had finished when the first bomb went off behind a row of national flags.
One 78-year-old runner, Bill Igriff, was blown to the ground and many bloodied spectators were pushed by the force of the blast through barriers onto the street. Igriff got up again and walked away with just scratches.
"We saw people with their legs blown off," Mark Hagopian, owner of the Charlesmark Hotel, told AFP from the basement of a restaurant where he had sought shelter.
"A person next to me had his legs blown off at the knee -- he was still alive."
Boston relived the horror in the many video films taken with telephone cameras that investigators also pored over in the hunt. Vigils and other ceremonies of remembrance were to be held in the city.
Armed National Guard troops and police patrolled Boston commuter trains and buses and authorities warned that tight security would be imposed for several days.
New York, Los Angeles, San Francisco and several other major US cities also put extra police on the streets. Obama was being kept informed of the investigation. A White House official said late Monday that the attack was being treated as terrorism.
"There were no intelligence warnings that we know of," said Representative Peter King, a member of the House Intelligence Committee who gave details of the huge hunt launched by US authorities.
"The war on terrorism is far from over, whether it is Islamic jihadists or right-wing extremists," he added.
Federal and state investigators and firefighters searched an apartment in the Boston suburb of Revere, Massachusetts, and took away bags after a man described as "a person of interest" was stopped.
A 20-year-old Saudi man who suffered a leg injury was under armed guard in hospital. But media reports said the man was cooperating and investigators said no suspect was in detention.
Russian President Vladimir Putin led a chorus of global condemnation, describing the twin explosions as "barbaric".
German Chancellor Angela Merkel said she was "horrified" and that "nothing justifies such a malicious attack on people attending a peaceful sporting event".
Iran strongly condemned the blasts which brought back memories of the nearly 3,000 people killed in suicide airliner strikes on the United States on September 11, 2001.
The New York Stock Exchange held a minute of silence before trading started for the day.
Organizers of Sunday's London Marathon said their race would go ahead despite the Boston attack.
British government officials including Home Secretary Theresa May met representatives from the security services to review Sunday's race through the streets of the British capital.
"We do have robust security measures in place... but, given events in Boston, it's only prudent for the police and the organizers of Sunday's race to re-examine those security arrangements," said London mayor Boris Johnson.