The FBI has briefed officials on their conclusion that the brothers likely used only the Internet to research bombcraft, multiple officials have told ABC News. Many skeptical senior officials in counterterrorism, however, note that the FBI was hampered by the Russian FSB security service in probing Tamerlan's 2012 trip to militant Islamist hotspots Chechnya and Dagestan, and that no one else has built the "Inspire" pressure cooker IED inside the U.S. homeland in the four years since the magazine's "How to build a bomb in the kitchen of your mom" recipe was made public.
Law enforcement sources say the Tsarnaevs also distinguished themselves by building four separate IED designs -- and used three types successfully, including pipe bombs and CO2 "cricket" grenades.
The FBI's fear of a follow-on attack last year by unknown accomplices led agents to press the surviving Tsarnaev brother for information without reading him his Miranda rights under a national security exception and while he was lightly sedated, prosecutors explained to the court in Wednesday's filing. Defense lawyers want his statements excluded from the case.
Interrogators questioned Tsarnaev 14 times over two days, taking breaks for rest, sleep and treatment periods, such as when they backed off questioning him for one 10 1/2-hour stretch. He admitted his involvement in the attacks to the FBI and denied there was a terror cell, the filing alleged.
Dzhokhar Tsarnaev has pleaded not guilty to federal terrorism charges.