BOSTON, Massachusetts: US investigators on Tuesday pledged to go "to the ends of the Earth" to find those behind the deadly Boston Marathon bombings, which President Barack Obama deemed an "act of terror."
Boston's Boylston Street, scene of the finish line carnage on Monday, remained sealed off as investigators sought leads in the worst attack in the United States since the September 11, 2001 atrocities.
Three people were killed and more than 170 wounded in the bombings. Police searched the apartment of one "person of interest" and a Saudi man remained under guard in hospital, but even Obama admitted there were no clear answers for a city in mourning.
"This was a heinous and cowardly act," Obama said at the White House. "Any time bombs are used to target innocent civilians, it is an act of terror."
"What we don't yet know, however, is who carried out this attack or why, whether it was planned and executed by a terrorist organization, foreign or domestic, or was the act of a malevolent individual," he said.
No suspect was yet in custody though several people were being questioned in the Boston area, but the hunt was expected to expand to other countries, police and Federal Bureau of Investigation (FBI) officials said.
"This will be a worldwide investigation," said Rick DesLauriers, head of the FBI's Boston office.
"We will go to the ends of the Earth to identify the subject or subjects responsible for this despicable crime," he added. "We are using full capacities of the FBI, to its fullest worldwide extent."
The two bombs, which were 13 seconds and about 100 meters (yards) apart, killed three people and injured 176, with 17 people in critical condition, Boston police commissioner Ed Davis told reporters.
The dead and injured were aged between two and 71 and included nine children.
Among the dead was an eight-year-old boy, Martin Richard, who had been waiting at the finish for his father to cross the line. His mother suffered grievous brain injuries and his sister lost a leg.
The bombs sent metal shrapnel flying into the crowd of thousands of people packing Boylston Street, ripping the limbs of many victims, doctors said.
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 in the 26.2-mile (42-kilometer) race had finished when the first bomb went off behind a row of national flags, sending out a powerful shockwave.
One 78-year-old runner, Bill Igriff, was blown to the ground and many bloodied spectators were pushed by the concussion 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 near the scene, told AFP. "A person next to me had his legs blown off at the knee -- he was still alive."
Boston relived the horror in the many videos taken with telephone cameras that investigators also pored over in the hunt. Police appealed for the public to send in pictures or videos.
While vigils and other remembrance ceremonies were to be held, 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.
"There were no intelligence warnings that we know of," said Representative Peter King, a member of the House Intelligence Committee.
"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, 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.
"We are interviewing a variety of witnesses right now in a variety of locations," said the FBI's DesLauriers, refusing to give details that could "compromise" the inquiry.
Russian President Vladimir Putin led a chorus of global condemnation, describing the twin explosions as "barbaric." German Chancellor Angela Merkel said "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 national flag over the White House and other US government buildings was lowered to half-staff, and the New York Stock Exchange held a minute of silence before trading started.
Organizers of Sunday's London Marathon said their race would go ahead despite the Boston attack, but security arrangements were under review.