"While the RC concept is similar, TEDAC assesses INSPIRE would not provide an individual with the appropriate details to translate these instructions for use with RC toy car components. Such construction would likely require previous knowledge of, or additional research into, RC toy car circuitry," a TEDAC analysis document said.
"They didn’t get it just from online and definitely didn’t get it just from reading Inspire magazine. So where did they get this training and how did they acquire the skill?" Barbero said.
The indictment of Dzhokhar Tsarnaev alleges that he and Tamerlan "armed themselves with five IEDs, a Ruger P95 9mm semiautomatic handgun, ammunition for the Ruger, a machete and a hunting knife" before engaging in their April 19 shootout with Watertown police. The bombs included a pressure cooker IED, three pipe bombs and a Tupperware tub filled with black powder and a fuse -- they exploded all but the tub during a firefight. The gun was bought illegally from a suspect still under investigation in another state, a law enforcement official told ABC News.
They also threw small black powder grenades called "crickets" at police during a car chase, sources said.
Allegedly creating and detonating such different IEDs put the Tsarnaevs in a class by themselves as post-9/11 homegrown terrorists, who typically have tried to perfect just one IED design and have rarely succeeded in detonating any at all, officials said.
"It is absolutely a major unanswered question of how they got that good and whether they had assistance of some kind," a senior counter-terrorism official told ABC News last week. "They had a better success rate than AQAP's master bombmaker al-Asiri," who created the underwear and printer cartridge bombs snuck aboard airplanes, which failed to detonate.
Ed Davis, who was Boston Police Commissioner at the time of the bombings, has his own doubts and told ABC News the devices were “technically challenging.”
“The questions that arise now [are] about what it took to put these together…” Davis said.
McCaul's staff recently completed a year-long investigation of the Boston attacks. It found federal counter-terrorism officials failed to alert Massachusetts law enforcement that Russia's security services in 2011 informed the FBI and later CIA that Tamerlan Tsarnaev and his mother Zubeidat Tsarnaeva were suspected of being radicalized Islamists.
"I think it's very clear today, after the investigation we conducted and the investigation the inspector general conducted of the intelligence community and the FBI that that foreign connection is real and that he was over there meeting with extremists," McCaul said. "He was greater radicalized and it’s very likely he did receive training."
An intelligence community inspector general report released last week revealed that the FBI only interviewed Tamerlan and his parents in Boston but not other associates or an ex-girlfriend he had assaulted. The FBI found nothing to support the Russian claim -- which stemmed from an intercepted phone call by Russian spy services.
"Now in fairness to the FBI, the Russians had information they didn’t share with the FBI after their warning came in and that’s unfortunate," McCaul said. McCaul and other officials were also quick to state that they see no evidence the Tsarnaevs were acting on orders of a foreign terrorist group. But extremists likely offered Tamerlan expertise, he agreed.
Tamerlan Tsarnaev soon faded as a counter-terrorism priority and he was able to travel to Chechnya and Dagestan in January 2012. He returned in July without facing further investigative scrutiny – partially because his name was misspelled in a security database -- and soon began posting jihadi material including videos on YouTube.
Davis said he believed it was “extremely important” for law enforcement to answer any lingering mysteries related to the bombing.
“We need to stay vigilant on these issues. We need to continue to ask questions and we need to make sure we have the best understanding possible of the situation so it doesn’t happen again,” he said. “[We] also have to understand the nature of the conspiracy… We have to understand as much as possible.”
FBI and Justice Department officials declined comment, citing Dzhokhar Tsarnaev’s upcoming death penalty trial.