(ABC News) -- Prosecutors seeking a death penalty conviction of accused Boston Marathon bomber Dzhokhar Tsarnaev on Wednesday said the FBI at the time of his arrest last year believed he and his brother Tamerlan had been trained by a terrorist group because of the sophistication of their weapons and tradecraft.
The Chechen immigrants are alleged to have built the two pressure cooker improvised explosive devices that detonated Monday, April 15, 2013 near the race's finish line, killing three Bostonians and wounding more than 260 other marathon spectators.
"These relatively sophisticated devices would have been difficult for the Tsarnaevs to fabricate successfully without training or assistance from others," the U.S. Justice Department said in a filing that opposed defense lawyers' efforts to have Dzhokar Tsarnaev's hospital bed confessions tossed out of the case.
Counterterrorism sources have said that investigators have been unable to find any hard evidence of any training or assistance.
But ABC News reported last month that many senior current and former counterterrorism officials -- including former Boston Police Commissioner Ed Davis, Homeland Security Committee Chairman Michael McCaul and former Joint IED Defeat Organization director Army Lt. Gen. Michael Barbero -- remain skeptical that the Tsarnaevs built their bombs solely from instructions they found posted online.
A side by side comparison last year of the pressure cooker IED design found in al Qaeda in the Arabian Peninsula's 2010 debut issue of "Inspire" magazine with evidence collected in Boston showed significant differences in the design and construction, according to analysis by the FBI’s Terrorist Explosive Device Analytical Center, ABC News reported last month.
On Friday, April 19, the brothers allegedly murdered an MIT cop and then engaged in a street gunfight with Watertown police officers that left Tamerlan dead. Hours later, the badly wounded Dzhokar was captured hiding inside a boat in a driveway.
Searches afterward of their residences, vehicles and other places they had frequented found no trace of the fine black powder siphoned from fireworks that were used in their IEDs, "again strongly suggesting that others had built, or at least helped the Tsarnaevs build, the bombs," the government filing stated.
Beyond the extraordinary skill required to build the pressure cooker bombs and three other types of rudimentary IEDs law enforcement sources say the Tsarnaevs threw at cops, the FBI saw the coordinated bombings, the recovery of only one of two remote-controlled switch-triggers used, and the rhetoric of Dzhokar's statements scrawled inside the boat all as hallmarks of al Qaeda, prosecutors recounted.
The brothers' use of "burner" cell phones with interchangeable SIM cards was viewed as further evidence that suggested they possibly "had received training and direction from a terrorist group," the court papers said.
The government did not say what investigators believe today.
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.
c has pleaded not guilty to federal terrorism charges.
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