The US launched a worldwide investigation into the Boston marathon bombings - the first terrorist attack on the American mainland since 9/11 - as the toll rose to three.
Two bombs, said to be pressure cookers packed with explosives and ball bearings, exploded near the finish line at around 2.50pm Monday (12.20am India time).
Authorities on Tuesday said the devices were similar to those used against American troops in Iraq and Afghanistan.
An eight-year-old boy was among the dead. Authorities put the number of wounded at 176, many with critical injuries.
"The FBI is investigating it as an act of terrorism," President Barack Obama said, adding that it wasn't known if it was committed by an individual, a "malevolent" individual or an organisation, foreign or domestic.
"This will be a worldwide investigation," lead FBI investigator Richard DesLauriers said at a briefing. "We will go to the ends of the earth to... bring them to justice."
Authorities searched the home of a Saudi national who was being treated in a Boston hospital for injuries suffered at the blast site. He has also been interviewed.
But police commissioner Ed Davis said he wasn't a suspect. There are no suspects or motives at present, the authorities insisted.
Among terror outfits, the al Qaeda is one of the usual suspects. Another, the Pakistan Taliban, has denied any involvement.
Law enforcement officers said they were pouring over hundreds and thousands of photographic evidence from an area that, according to Davis, had the country's most surveillance cameras and was the most complex crime scene in the history of the department.
The blasts were the first terror attack on American soil since September 11, 2001, though there have been many on US facilities abroad, specially its diplomatic missions.
The anniversaries of two of America's worst cases of domestic terrorism and mass violence - the Waco Massacre and the Oklahoma bombing- fall later his week on April 19.
Tim McVeigh had bombed a federal building in Oklahoma city on April 19, 1995, killing 168, partly in rage over the Waco massacre two years ago.