The bombers who attacked the Boston Marathon last week did not rely on cellphone detonators, but rather toy car speed controllers as the trigger, requiring a clear view of the explosives, according to a national security source familiar with the FBI investigation.
The source, who agreed to discuss progress in the marathon bombing case on the condition of anonymity, said the finding is significant, adding the use of remote-control toy parts as a detonation mechanism is not found in the Al Qaeda online magazine Inspire, which was cited in early reports as the suspects' likely bomb-making guide.
While the working theory among investigators is that one or both of the Tsarnaev brothers triggered the detonation using the speed controllers, Fox News is told a third party in the crowd has not been ruled out, though there is no evidence suggesting a third party at this time.
"I can't comment on any of the specifics on the design of the weapon that went off, but it is very clear when you take the totality of it that there was some outside counsel to these individuals on how to build and how to detonate," Republican Rep. Mike Rogers, chairman of the House Intelligence Committee, said.
Asked about possible third-party involvement, Rogers did not rule it out.
"This was not something that we believe that on their own that they came up with, that design was on their own," he said. "That's why those six months in Russia becomes so important. And other persons of interest that I know investigators would like to talk to becomes very, very important here."
On Thursday, a senior counterterrorism official from the New York Police Department also knocked down reports that the bombs could be built by amateurs using information gleaned from the Internet.
"These are not crude bombs, these are very effective small bombs," Richard Daddario, deputy commissioner for counterterrorism explained to the House Homeland Security Subcommittee. "And I think people shouldn't use the term ... crude device."
Suspect Dzhokhar Tsarnaev, 19, told investigators from his hospital bed that he and and his brother, Tamerlan Tsarnaev, 26, used a guide from Al Qaeda's online magazine in making the pressure-cooker explosives that killed three and injured more than 200 people in Boston.
But the national security source noted that remote control parts are not mentioned in the bomb-making manual published in Inspire -- a magazine that is distributed mainly through online jihadist chat rooms, some of which are password-protected.
Catherine Herridge is an award-winning Chief Intelligence correspondent for FOX News Channel (FNC) based in Washington, D.C. She covers intelligence, the Justice Department and the Department of Homeland Security. Herridge joined FNC in 1996 as a London-based correspondent.