(CNN)It's the lingering mystery that haunts the FBI and others in law enforcement: Did the Tsarnaev brothers orchestrate the Boston terror attacks alone?
Did the Boston terrorists get help building bombs?
The bombs themselves were built to kill, a complicated recipe of destruction that was available online but difficult to assemble.
In a motion filed last May, federal prosecutors asserted that interviews with Dzhokar Tsarnaev "provided reason to believe that the Tsarnaevs had accomplices."
One year after the explosions at the marathon's finish line, then-Boston Police Commissioner Ed Davis told CNN he suspected the brothers had help to carry out their plan.
"They're complex devices and the fact that these devices went off in such close proximity showed some significant planning," Davis said. "The fact that they were able to pull this conspiracy off and kill and hurt so many people you have to look at that very closely, there's a lot going on here and it has to be fully vetted."
"Someone who might have either assisted in making the bomb, perhaps helped test it, perhaps get the components -- that's part of the investigation that's still ongoing, because they've never closed those questions," said CNN's justice reporter Evan Pérez.
Just days after the brothers carried out their deadly plans, CNN's "The Lead with Jake Tapper" spoke to taxi driver Jim Duggan who said he picked up the would be terrorists at the train station the day before the attacks, heavy backpacks in tow.
"I don't know if it was a pressure cooker bomb or a pipe bomb but honestly it was a lot heavier than some wet towels and sneakers from the gym," said Duggan.
"And then the questions started to roll through my mind, well wait a minute. What were they doing here at the train?" he said.
Where were they coming from? With whom had they met? Duggan estimates the bags he helped them lift weighed nearly 30 pounds, and the brothers were very assertive that they did not want him to touch them.
And there may have been more attacks planned.
"He told the FBI, apparently, that he and his brother had intended to drive to New York and detonate additional explosives in Times Square," former New York City Mayor Michael Bloomberg said in 2013.
Prosecutors say Dhokar destroyed disposable cell phones before he was eventually caught. Why? Who had he called that he did not want anyone to know about?
These future plans and communication tactics lead prosecutors to believe "others might have radicalized them, directed them [or] trained them, and that others might be planning or poised to carry out additional attacks."