This Wednesday, 21-year old Dzhokhar Tsarnaev will be formally sentenced to die for the deaths and injuries he inflicted at the Boston Marathon and on the MIT campus. Neither side in his trial disputed his role. But by the end of his trial, there remain unanswered questions surrounding the attack; including did the Tsarnaev brothers act alone? Our report begins near the finish line on April 15, two years ago.

They were neither the fastest runners nor the slowest. They were the middle of the pack and heading toward the finish line one by one as the clock on Boylston Street struck –2:49 pm. Some marathon watchers on that sun-drenched Monday crowded in front of the Forum restaurant, and suddenly all heads except one turned toward the sound of the loud boom a block away:

Law enforcement officials were soon focused on a single suspect, but after examining hundreds of photos and video surveillance tape the FBI’s Richard DeLaurier announced a second. “They are identified as suspect one,and suspect two,” DeLaurier told reporters during a press conference that day. “They appear to be associated. Suspect one is wearing a dark hat. Suspect two is wearing a white hat.”

Over the next 24 hours Boston, Watertown and Cambridge were awash in a sea of blue flashing lights as police responded to the murder of MIT officer Sean Collier, pursued the suspects in a frantic car chase through Watertown, exchanged gunfire, dodged explosives and finally caught the remaining suspect hiding out in a dry-docked boat. Tamerlan was dead and Dzhokhar was later convicted and sentenced to die. Case closed. But a question then and now remains unanswered for many: Did the Tsarnaev brothers act alone?

One month after the penalty was announced, Boston Police Commissioner William Evans told me it was those two and those two only.

“You know I seen the video,” Evans says. “I seen these two kids from the get-go, I seen it Wednesday before we even captured them, those two individuals walking down Boylston. I seen, uh, who I call ‘white hat’ and ‘black hat’. White hat put that behind the Richards family. I wasn’t really concerned who could have been behind it. I was very confident we got the two people who were so sinister that they would do something like that.”

And Commissioner Evans has heard the doubts: “I know there’s a lot of speculation out there,” he said.”The widows and the girlfriends and everything else, but ultimately it comes down to the two people who pulled off the act and I’m satisfied with whom we got.”

But other close observers of the bombing investigation and subsequent trial believe that several aspects of this case remain jagged, fragmented and completely unsatisfactory. Where were the bombs made? Where were the bombs tested? Were the Tsarnaevs financed by someone else still out there? Did the Tsarnaevs act alone?

“Well to my mind, that’s the biggest question. And that’s the scariest question,” says Masha Gessen, a New York-based Russian-American journalist and author of “The Brothers",  which examines the lives and motives of the Boston Marathon Bombers.

Gessen says “We heard an FBI agent testify during the trail that the FBI still doesn’t know where the bombs were made.”

FBI agents who took the stand during the trial talked about how dirty the gun powder was such that it would have left residue. But few traces were found at Tamerlan’s 410 Norfolk street apartment in Cambridge, according to court documents.

“The question of the bombs themselves have never been fully answered,” Gessen says. “In a US Attorney’s motion to the federal court in the trial of Dzhohar Tsarnaev, the US government said we actually do not think that the bombs were built in Tamerlan’s apartment.”

Kade Crockford of the ACLU of Massachusetts —paraphrasing testimony from the Tsarnaev trial said “We scoured the place and we found very little residue of black powder. We suspect that they were built elsewhere.”

Evidence during the trial also demonstrated that the bombs were not built in Dzhohar’s UMass Dartmouth dorm room. Crockford is the director of the technology for liberty program at the ACLU of Massachusetts and led the organization’s investigation into the Boston Marathon bombing Her concerns are echoed by author Masha Gessen:

“Without knowing who the space belonged to, we don’t know if person or persons—were involved unwittingly,” says Crockford. “Did someone let Tamerlan use his garage or whether there was a full-fledged accomplice? Consider the scale of what they were able to accomplish in a short time—the number of bombs they built—using firecrackers, which is an incredibly time intensive process from what I understand, and they managed to do this in a short space of time, that also raises the question of whether there were other people involved.”

Inside a secure facility at Northeastern University, Dr. Adam Hall, shows me how a spectrometer works. It’s an instrument used to measure wavelengths of light spectra.

I’m here to speak with Hall about bombs and bomb making. In addition to being a chemist and academic, Hall is also a former forensics expert for the Massachusetts State police. He shows me photographs of some of the various types of explosive devices he has examined during his career.

Hall believes that that one or both of the Tsarnaevs could have built the bombs on their own.

“Unfortunately,” he says “there is information out there through the internet and other sources that provide some information and guidance on construction of devices.”

But says Hall, “What was unique about this in terms of domestic US based investigations is the use of the pressure cooker, which is a bit more common in other areas of the World. Investigators on the federal level will rarely see something like this on US soil.”

At 410 Norfolk Street in Cambridge, FBI agents testified that at they found Christmas lights, which of course is an unusual find in a Muslim household. We learned on March 24 during the trial that they were used as a triggering device for the bombs. However, an FBI specialist on the stand also was not able to explain to the jury where the remote for the bomb came from or how it was made. Adam Hall says “It certainly shows some level of knowledge. Where that knowledge came from is difficult to say.”

But Hall says there is still no smoking gun in terms of answering the question—Did they Act Alone? After all, on a Tsarnaev computer, investigators found an on-line al-Qaeda manual with step by step instructions “how to make a bomb in the kitchen of your mom”—according to testimony in the trail.

Hall wonders how the bomb makers were able to accrue the enormous amount of gunpowder deemed necessary to turn kitchen utensils into deadly explosives. To set off the bombs would have required approximately 20 pounds of powder. The government intimated during the trial that the gunpowder came from firecrackers, but the amount needed says Hall would have made extrapolation very difficult, and it should have left a mess whereever it was made.

“Black powder can be purchased commercially, but it’s not as available now a days as it was years ago,” says Hall. “If it had been obtained from fireworks it would be a very costly approach to harvesting the black powder from the fireworks to obtain enough to charge a device as large as a pressure cooker.”

And how did they finance what was clearly a low budget operation, but collecting and dismantling firecrackers would still required a good deal of money? Hall says “It would not be the most cost effective or inexpensive approach."

What concerns Hall are the same questions that concern others we interviewed: Where were the bombs made? Where were they tested? If they were made somewhere other than the two Tsarnaev residences, did they have help? Did they act alone?

The tragedy of the bombing itself, the sheer horror that Bostonians are trying to ease into the background becomes more difficult when questions linger–-as the song goes, like a haunting refrain.

Masha Gessen says “the fact that the FBI comes right out and says that they don’t know where the bombs were made, and that kind of passes muster, is mind-boggling to me.”

Kate Crockford of the ACLU says as much as the public understandably is ready to move on, she still has many questions.

“If someone else did help these brothers build these bombs,” Crockford says “that person is still out there and knows how to build bombs. And so for the people of Boston, despite the fact that we have now closed at least first chapter in what is bound to be a very long drawn out appeals process, people are still scratching their heads about some pretty significant aspects of this I think.”