US investigators have released new evidence linking pressure cooker bombs to the Boston Marathon blasts. But it remains unclear who was behind the attack that left three people dead and injured dozens more.
New images released by the Federal Bureau of Investigation (FBI) late on Tuesday show what appears to be a mangled pressure cooker and black bag at the scene of the Boston bombings.
Richard DesLauriers, FBI agent in charge in Boston said fragments of suspected pressure cookers thought to have been used to pack the bomb were being put together by experts. He added that metal pellets and nails had also been recovered.
"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 more than 1,000 officers were working on the case in Boston alone.
US President Barack Obama has announced he will attend a special service Thursday for those killed and injured in the twin-bombings near the finish line of the Boston Marathon on Monday.
The victims have been identified as eight-year-old Martin Richard and Krystle Campbell, 29, a restaurant manager from Massachusetts. According to China's Xinhua news agency the third victim was a Chinese national believed to be a graduate student at Boston University. Her name has not yet been given.
More than 1,000 people attended a candle-lit vigil for Martin Richard near his family home in the Boston suburb of Dorchester on Tuesday. Hundreds more attended another vigil in central Boston and a church service near the scene of the blasts.
A further 170 people were also wounded in the bombings, 17 of them critically. Dozens are still undergoing treatment in Boston hospitals where doctors are believed to have carried out at least 13 amputations.
'Act of terror'
Speaking at a press conference on Tuesday Obama said the bombings were now being treated as an act of terrorism.
"This was a heinous and cowardly act," Obama said in Washington. "Any time bombs are used to target innocent civilians, it is an act of terror."
He said it was unclear if a foreign or domestic group was behind the attack, or if it was the work of a "malevolent individual."
Obama praised the first responders who helped treat the victims in the initial moments after the explosions and the medical staff who continue to treat the wounded.
"If you want to know who we are, who America is, how we respond to evil - that's it," he said. "Selflessly, compassionately, unafraid."
Calls for photos
A few hours before Obama's remarks, a press conference was held in Boston featuring heads of the agencies taking part in the FBI-led investigation. Multiple reminders were issued that the public is encouraged to submit pictures of video of the blasts and the race to give authorities a better idea of what happened.
"This is probably one of the most photographed areas in the country yesterday," said Boston Police Commissioner Ed Davis.
Officials were also able to reassure the public that there are currently no threats to public safety, and that the two bombs that exploded at the finish line were the only two involved in Monday's attacks.
Massachusetts Governor Deval Patrick said that there were "no unexploded explosive devices found." Other suspicious packages that had been checked out turned out to be harmless.
The two explosions occurred 50 to 100 yards (45-91 meters) apart shortly before 3 p.m. local time (1900 UTC) near the north side of Bolyston Street.
The annual Boston marathon attracts over 20,000 runners and about 500,000 spectators annually. A local state holiday commemorating the first battles of the American Revolutionary War, known as Patriots' Day, coincides with the sporting event.
ccp/jm (AFP, AP, Reuters)