US investigators zeroed in Wednesday on the suspected pressure cooker bombs that exploded at the Boston marathon, killing three people and injuring more than 180, as they hunted for perpetrators.

The FBI released photographs of mangled metal, the apparent remnant of a pressure cooker, and of a shredded black backpack, where it said one of the bombs was stashed before it exploded, spitting nails and metal pellets into the crowds, causing grisly injuries and deaths.

With no claim of responsibility made and police not committing to blaming foreign or domestic militants, Boston harbored widespread questions about the perpetrator or perpetrators as the city and the nation paid tribute to the dead.

US President Barack Obama condemned Monday's attack at the marathon finish line as "an act of terror." He will attend a special service for the victims in Boston on Thursday.

Two days after the attack, evidence was still being collected and rushed to the Federal Bureau of Investigation's laboratory in Quantico, Virginia for a thorough analysis.

"The scene is going to take several days to process," said Gene Marquez, acting special agent in charge of the Bureau of Alcohol, Tobacco, Firearms and Explosives office in Boston.

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

"This will be a worldwide investigation," 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.

DesLauriers said similar, easy-to-make devices are used as roadside bombs in Iraq and Afghanistan, adding that "the range of suspects and motives remains wide open."

Two US officials said so far there was no indication that Al-Qaeda or other foreign extremist organizations were behind the attack, but they added the investigation was still at an early stage.

"It's too early to draw any conclusions," one US official, who spoke on condition of anonymity, told AFP.

Doctors, who carried out at least 13 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 George Velmahos, head of emergency medicine at Massachusetts General Hospital.

The two bombs, which exploded 13 seconds and about 100 meters (yards) apart, sprayed the shrapnel into the crowd of thousands of people lining Boylston Street to watch the runners cross the finish line. Three people were killed and at least 180 injured, according to the latest toll. Some 17 people were in critical condition. The dead and injured were aged between two and 71 and included nine children.

Among the dead was eight-year-old Martin Richard, who was watching the marathon with his family. His mother suffered a grievous brain injury and his sister lost a leg.

A Chinese graduate student at Boston University also died, the university and the Chinese consulate in New York confirmed. Her family requested her name not be released, her name, Lu Lingzi, was being widely circulated in US and Chinese media.

The third fatality was named as Krystle Campbell, a 29-year-old restaurant manager from Massachusetts. A candle-lit vigil for Richard drew 1,000 people to a park near the family home in the Boston suburb of Dorchester. Hundreds attended another vigil in central Boston and a church service near the scene of the blasts.

Most of the 23,000 runners in the 26.2-mile (42-kilometer) race had finished when the bombs went off.

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.

Armed National Guard troops and police patrolled Boston airport, commuter trains and buses and authorities warned that tight security would last several days, particularly as Obama was to be in the city on Thursday.

"There were no intelligence warnings that we know of," said Representative Peter King, a member of the House Intelligence Committee, who highlighted that it was not known whether foreign or domestic attackers planted the bombs.

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.

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