Updated at 8:20 p.m. ET
The top Democrat on the House Intelligence Committee said it was a remote control for a toy car that triggered the bombs in the Boston Marathon explosions that killed three people and wounded more than 260, CBS News learned.
Rep. Dutch Ruppersberger (D-MD) made the announcement early Wednesday evening.
Earlier, CBS News investigative senior producer Pat Milton reported that a radio-controlled type of device, like those commonly used with toy cars, or components of such a device might have been used to trigger the bombs, according to the source.
The source said analysis of the bombs remains ongoing.
Two officials on Wednesday told The Associated Press the bombs were not very sophisticated. One of the officials described the detonator as "close-controlled" meaning it had to be triggered within several blocks of the bombs.
Both U.S. officials are close to the ongoing investigation but spoke on condition of anonymity because they were not authorized to discuss it publicly.
It was not immediately clear what the detonation device was.
Also, two U.S. officials say Dzhokhar Tsarnaev, the surviving suspect, was unarmed when police captured him hiding inside a boat in a neighborhood back yard. Authorities originally said they had exchanged gunfire with Dzhokhar for more than one hour Friday evening before they were able to subdue him.
The officials, speaking on condition of anonymity because they were not authorized to discuss the ongoing investigation, say investigators recovered a 9 mm handgun believed to have been used by Tamerlan, from the site of a gun battle Thursday night, which injured a Massachusetts Bay Transportation Authority officer. Dzhokhar was believed to have been shot before he escaped.
The officials tell The Associated Press that no gun was found in the boat. Boston Police Commissioner Ed Davis said earlier that shots were fired from inside the boat.
Meanwhile, CBS News correspondent Bob Orr reported that investigators also believe the brothers helped finance their plot through drug sales. Sources say Tamerlan Tsarnaev, who was unemployed, made money selling marijuana.
A criminal complaint outlining federal charges against Dzhokhar Tsarnaev described him as holding a cellphone in his hand minutes before the first explosion.
Cellphones have been used to trigger bombings in war zones.
Authorities are trying to determine whether Dzhokhar Tsarnaev and his older brother Tamerlan Tsarnaev, who died in a shootout with authorities, used gunpowder from fireworks to make the bombs they're accused of planting by the Boston Marathon finish line, CBS News correspondent Bob Orr reports.
Just 10 weeks before the attack, Tamerlan Tsarnaev walked into a New Hampshire fireworks store looking for something powerful.
"I do remember just him asking, well first him having the accent and then him asking for the biggest loudest stuff, and I showed him one other product, but he definitely stressed that he wanted the biggest loudest stuff," cashier Megan Kearns said.
She said that Tsarnaev spent about 10 minutes in the store before paying in cash for two "Lock and Load" mortar kits, each containing 24 pyrotechnic shells. He paid $200 for one box and got the other free under a store promotion.
Both explosive devices appear to have been placed in sealable metal pots called pressure cookers and packed with nails and ball bearings designed to amplify the damage from the explosions, Orr previously reported.
Investigators believe the bombs were hidden in black nylon backpacks and housed inside the pressure cookers. Pressure-cooker bombs can help boost the power of relatively small devices by briefly constraining the blast. When the cookers do explode, they can add large chunks of metal to the shrapnel spray.
CBS News senior correspondent John Miller previously reported that police think the brothers killed a Massachusetts Institute of Technology campus police officer for his weapon while they were the subjects of last week's massive manhunt.
The brothers only had one real gun and one pellet gun when they were on the run Thursday, Miller reports. Investigators now believe that Officer Sean Collier was killed Thursday because the two bombing suspects wanted to take his gun.
Investigators believe because the officer's holster had a locking system, they apparently couldn't get the gun out.
Collier was shot in the head execution-style while sitting in his patrol car.
On "CBS This Morning" Wednesday, Miller reported that the FBI legal attache assigned to the U.S. Embassy in Moscow has joined with the Russian security services, the FSB, to work on a joint investigation, tracking down people, retracing steps Tamerlan Tsarnaev took during recent trips to Chechnya and Russia and interviewing people.
"What they're looking for is two things: Was there a bombmaker, a bomb teacher, a bomb test that happened there?" Miller said.
In his questioning in the hospital, Dzhokhar Tsarnaev said they were self-taught and self-radicalized.
"But you don't take that at face value, so you go back over that trip," said Miller. "Remember the younger brother may not know what his brother did in the Russian trip because he didn't go."
Miller said a Muslim convert known to the Tsarnaev family only as Misha, who family members told the AP steered Tamerlan Tsarnaev toward a strict strain of Islam, probably wouldn't be the bombmaker or teacher. However, he may be the person who connected Tamerlan Tsarnaev with one.
"Those are exactly the kinds of leads they're running backwards," said Miller. "They want to talk to everybody he met on that trip."