The Boston Marathon explosions that killed three people and wounded more than 260 were triggered by a remote-controlled detonator.
The bombs were not very sophisticated and were set off within several streets away because the detonator was a "close-controlled" device, according to two US officials who spoke to AP news agency on condition of anonymity.
It was not immediately clear what the detonation device was.
A criminal complaint outlining the charges against surviving 19-year-old bombing suspect Dzhokhar Tsarnaev described him as holding a mobile phone minutes before the first explosion.
The two sources, one reportedly briefed by the FBI and the other close to the investigation, also told AP that Dzhokhar had told interrogators that he and his brother were angry about US wars in Iraq and Afghanistan.
They also said a 9mm pistol, believed to have been used by suspects in a gun battle with police last Friday had been recovered. Its serial number had been scratched off.
Boston Police Commissioner Ed Davis has said more than 250 shots were fired in the first shootout in Watertown during which explosives were thrown at police and 26-year-old Tamerlan Tsarnaev died.
His younger brother, who escaped, was captured some time later hiding in a boat in the same residential suburb of Boston after a gun battle with police.
He remains in hospital and is being questioned about the marathon attacks. He has been charged with using and conspiring to use a weapon of mass destruction.
It has also emerged the Russian government was in touch with the CIA with concerns about Tamerlan Tsarnaev. Until now, only contact with the FBI had been acknowledged by US officials.
Details of the ongoing investigation into the bombings emerged as a memorial service was held for Sean Collier - a university police officer who authorities say was killed by the Boston Marathon bombing suspects.
Up to 10,000 Massachusetts Institute of Technology students, faculty and staff as well as law enforcement officials from across the US gathered at Briggs Field to pay solemn tribute to the campus officer.
Vice President Joe Biden, as well as members of Mr Collier's family, also attended the event which was not open to the public.
Mr Biden, who grew up in the same neighbourhood as the Collier family, said the tragedy had brought him to "tears" after relatives painted "a vivid picture" of their loved one.
He told the congregation: "Of all the things I've read, not knowing Sean, what struck home the most to me was that a student was quoted as saying, 'he loved us and we loved him'. What a remarkable son, what a remarkable brother."
Mr Biden described the Tsarnaev brothers, the only suspects in the Boston attack, as "two twisted, perverted, cowardly, knock-off jihadis".
In an emotional and passionate speech, he added: "What makes me so proud to be an American is that we have not yielded to our fears.
"The irony is, we read about these events, we experience them, but the truth is, on every frontier, terrorism as a weapon is losing. It is not gaining adherents, and what galls them the most is America does remain that shining city on the hill."
Mr Collier had only been in the job for a year when he was fatally shot in his squad car at the beginning of the dramatic manhunt for the suspects, four days after the twin blasts at the Marathon.
The 26-year-old was well-respected by his colleagues and was popular with students, and often went on hikes with the MIT student outing club.
MIT police chief John DiFava said: "What made Sean good? There are many reasons. But I believe the most important is the fact that he was the same person in uniform that he was when he wasn't wearing the uniform.
"His care and compassion was genuine ... and because of his depth of character he was able to achieve a level of trust with people of all backgrounds that was truly remarkable. I am sure his love of life and that mischievous grin added to his ability to connect with students."
He added: "Sean, we love you and we'll never forget you."
Wednesday's gathering followed a private funeral mass for Mr Collier on Tuesday.
Meanwhile, US diplomats have travelled to Russia's Dagestan region, a largely Muslim area on the Caspian Sea, to interview the parents of the bombing suspects.
Their father Anzor Tsarnaev and mother Zubeidat Tsarnaeva have reportedly said they will be travelling to the US on Thursday to visit their surviving son and bring his dead elder brother's body back to Russia.
They have refuted the allegation that their sons carried out the last Monday's bombings and have said their sons were framed.
Some relatives have claimed Tamerlan fell under the influence of a mystery Muslim convert - a red-bearded man known to the family only as Misha - and was steered towards an extreme form of Islam.