Nearly two years ago on Marathon Monday, the second-tier of runners made their way to the finish line, and an 8-year-old boy, Martin Richard, was cheering them along. His cheers ended abruptly as did those of two other victims, along with 260 injured bystanders and runners in that first chapter of the Marathon bombing. Chapter Two was the manhunt, shootout and capture. Chapter three begins Wednesday: The federal trial of the surviving suspect, Dzhokhar Tsarnaev.

Over the next four months during the Boston Marathon bombing trial, the tragedy of that day will be replayed over and over again in agonizing detail.

We will hear evidence presented about pressure cooker bombs hidden in knapsacks and the alleged bomber hiding in a boat, we will see video and photographic images of the suspect, horrific and sad stories about limbs blown off and lives shattered, and about resilience symbolized in "Boston Strong."

The principal legal questions: Did Tsarnaev plant a bomb that killed three and injured dozens? Did he help murder MIT police officer, Sean Collier? But an existential question — one which both the defense and prosecution will attempt to answer: Who is Dzhokhar Tsarnaev?

In his East Cambridge neighborhood he was the Russian kid with the friendly smile passing by Troy Anthony’s barbershop each weekday morning.  Troy Anthony smiled back. 

To Luis Vasquez, his soccer coach at Cambridge Rindge, Tsarnaev wasn’t exactly a star athelete:

"He wasn’t very good," Vasquez said, but  "he brought a hard work ethic to the team."

Alice LoCicero — a clinical psychologist and researcher with the Center for Multicultural Training in Psychology at Boston Medical Center, and author of "Why 'Good Kids' Turn Into Deadly Terrorists" — said the prosecution's contention that the older, now deceased Tsarnaev brother, Tamerlan, had undue influence over Dzhokhar, is "plausible."

Prosecutors have asked Judge George O'Toole to reject this argument. To answer the question of who Dzhokhar Tsarnaev is from their perspective, U.S. prosecutors may call on Tsarnaev’s embattled friends from the University of Massachusetts Dartmouth.

Dias Kadyrbayev and Azamat Tazhayakov, two Kazakh nationals, and a Cambridge man, Robel Phillipos, were all convicted last year of removing incriminating evidence or lying to investigators. There has been speculation that one or more of the men will be called to testify for the prosecution.

"He knows nothing about Dzhokhar Tsarnaev except that he’s absolutely furiously angry at a man who he believes now killed and maimed innocent people," Phillippos’ lawyer, Susan Church, said on Greater Boston last October. " … He doesn’t know anything. He never knew anything about Tsarnaev being a radical."

Another possible witness in this trail may never actually take the witness stand: That’s former Boston FBI head Ricahrd DeLauriers, who showed photos of the suspected Marathon bombers on live, national telelvison —  a dramatic picture that U.S. prosecutors have asked the court to allow them to introduce during the trial. The defense described the photo and its introduction into the trial as inappropriate.

At a late afternoon press conference on April 18, DesLauriers told reporters that the persons being sought should be considered dangerous. Prosecutors described the men who fashioned shrapnel-filled pressure cookers, hidden in backpacks, set off by timers, and loaded with nails and ball bearings as individuals willing to inflict severe harm on innocent people.

So, again, who is Dzhokhar Tsrnaev ?   An affable but off-the-rails 20-year-old who smoked dope, failed in the classroom and was easily led by an overbearing brother?

Or is Tsarnaev a stone-cold, non-remorseful terrorist who misled friends and conspired with his brother to kill as many people as possible to avenge Muslims in Iraq and Afghanistan, according to what police alleged he scribbled on the wall of the Watertown boat where he was found?

Whoever he is, his story will be spun by defense and prosecutors to their respective advantage during the trial — this third chapter in the Boston Marathon bombing case. The final chapter will be predicated on a guilty verdict and whether Tsanaev is subsequently allowed to live or sentenced to die.