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(CNN) – It's a question with massive implications for national security: Did Boston Marathon bombers Dzhokhar and Tamerlan Tsarnaev act alone?
Or could they have had help, either from abroad, or inside the United States, when they set off two bombs at the finish line of the 2013 Boston Marathon?
In a motion filed Wednesday, prosecutors are now arguing that those bombs, which included black powder from hundreds of emptied out fireworks, were too "sophisticated" for the Tsarnaevs to have built on their own without "the training or assistance from others."
Former Boston police commissioner Ed Davis echoed that point when CNN's Jake Tapper spoke to him about it a few weeks ago.
The prosecutors also say that searches of the Tsarnaev's residences and vehicles didn't show any sign of black powder.
So where were the bombs built, and who might have helped build them?
Last year, while reporting on this story from Boston, CNN spoke with cab driver Jim Duggan, who says he picked up the Tsarnaevs one day before the explosions at the finish line, saying the brothers had just gotten off a train in Malden, Massachusetts, and had two bags with them.
Richard Clarke, former White House national coordinator for security and counterterrorism for the Bush and Clinton administrations, says two key pieces of information are missing.
"What did Tamerlan Tsarnaev do when he was in Chechnya? ... We don't know. The second gap is where were the bombs put together?" Clarke said in an interview with CNN's "The Lead with Jake Tapper."
While the former counterterrorism official says it's "conceivable" the Tsarnaevs had outside training and assistance, "the bomb bears a resemblance to the design in the online terror magazine 'Inspire.'"
Dzhokhar Tsarnaev wrote a note after the standoff with police that left his older brother dead, writing he was jealous of Tamerlan, but "I do not mourn because his soul is very much alive."
"God has a plan for each person. Mine was to hide in this boat and shed some light on our actions.... We are promised victory and we shall surely get it," he wrote.
"These are the words of a committed jihadist, these are the words of a fundamentalist Islamist terrorists," said Clarke.
"It doesn't look like a lot of his activities in the months leading up to the attack were those of a fundamentalist Islamist. But those are familiar words, which means he was converted at some point. Who converted him?" said Clarke.
Some are converted online, but others are converted by very persuasive imams, says the former counterterrorism official.
"So it's possible that there's someone in the United States that helped bring him across the line, helped convert him into being a terrorist," said Clarke.