BOSTON, Massachusetts (AFP) - US investigators said the range of suspects and motives in the grisly Boston bombings remained "wide open" as experts assessed remnants of the crude devices designed to inflict maximum suffering.

US President Barack Obama condemned Monday's attack at the finish line of the city's marathon, which killed three people and wounded more than 170 others, as "an act of terror."

Mr Obama, who will attend a special service for the victims in Boston on Thursday, said there was still no indication who carried out the attack, and the Federal Bureau of Investigation said no claim of responsibility was made.

Boston's Boylston Street, scene of the carnage, remained sealed off as investigators sought leads in the worst attack on civilians in the United States since the September 11, 2001 atrocities.

US authorities threw virtually every investigation agency into the hunt with more than 1,000 officers working in Boston alone, said Mr Rick DesLauriers, head of the FBI's Boston office.

"This will be a worldwide investigation," Mr DesLauriers told reporters. "We will go to the ends of the Earth to identify the subject or subjects responsible for this despicable crime," he added.

He said fragments of "possible" pressure cookers used to pack the bomb had been found and were being put together by experts. He added that BB pellets, nails and shreds of black nylon from bags had also been recovered.

Similar easy-to-make crude bombs are used in Iraq and Afghanistan.

"The range of suspects and motives remains wide open," he said later in the day, adding: "But rest assured we're working hard to get the answers."

"Someone knows who did this," he said, urging the public to come forward with any relevant information. No one was in custody so far.

Doctors, who carried out at least 10 amputations, some at the scene, gave details of the bomb impact.

"These bombs contained small metallic fragments more consistent with pellets and other small pieces of metal, but also spiked points that resembled nails without heads," said Mr George Velmahos, head of emergency medicine at Massachusetts General Hospital.

Mr Roger Walls at Brigham and Women's Hospital said a dozen carpentry nails were taken from the body of one victim.

The two bombs, which were 13 seconds and about 100 m apart, sprayed the shrapnel into the crowd of thousands of people lining the Boylston Street to watch the runners cross the finish line.

Three people were killed and 176 injured, 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.

Massachusetts resident Krystle Campbell, 29, was also named as one of the dead. The third victim was a Boston University graduate student whose name has not yet been given.

Ms 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 42-km race had finished when the first bomb went off, sending out a powerful shockwave.

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 airport, commuter trains and buses. Authorities warned that tight security would be imposed for several days.

Federal and state investigators searched an apartment in the Boston suburb of Revere, and took away bags after a man described as "a person of interest" was stopped.

"There were no intelligence warnings that we know of," said Representative Peter King, a member of the House Intelligence Committee, who added that investigators had not determined whether foreign or domestic groups were involved.

New York, Los Angeles, San Francisco and several other major US cities boosted security as Russian President Vladimir Putin led 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 also strongly condemned the blasts.

The national flag over the White House was lowered to half-staff and the New York Stock Exchange held a minute of silence before trading started.

Organisers of Sunday's London Marathon said the race would go ahead, but security arrangements were under review.