April 22, 2013 -- intro: A suspect in the Boston Marathon bombing is dead and the other has been captured and charged, but many questions are swirling around the suspects and their actions. Authorities hope that hospitalized suspect Dzhokhar Tsarnaev, 19, may be able to give the some answers but he has so far only been able to communicate through writing.
Here's a breakdown of what we know--and don't know--so far.
quicklist: 1title: What was the motive for the terrorist attack?text:No one knows.
Family and friends have been shocked to hear that the Tsarnaev brothers could be responsible for the deadly terrorist attack, especially Dzhokhar who has been described as "very American." But red flags have been popping up in the older brother's background, including concerns about his increasingly extremist views on religion. In 2011, he was looked into by the FBI on behalf of a foreign government.
Tamerlan Tsarnaev, 26, went to the restive Russian province of Dagestan for six months, and authorities are trying to determine if that propelled him to Islamic militancy, but so far it is only a question.
quicklist: 2title: Was the carjacking victim freed or did he escape?text:This question was answered today.
The intense hunt that led to the killing and capture of the two Boston Marathon bombing suspects began the night of April 18, three days after the terrorist attack, when the Tsarnaev brothers allegedly ambushed and killed a Massachusetts Institute of Technology police officer and then carjacked a black SUV.
Authorities initially reported the Tsarnaev brothers released the carjacking victim after driving around with him for a while, but information in the newly released criminal complaint against Dzhokhar Tsarnaev indicates that the carjacking victim escaped.
"After being carjacked by one of the brothers and then driving to pick up the other, the victim was forced to hand over his ATM pin information and was eventually driven to a gas station where he managed to escape when the two men got out of the car, according to the complaint," the complaint states.
The carjacking victim has not been identified.
quicklist: 3title: Why was MIT police officer Sean Collier murdered?text:One of the most perplexing pieces of the puzzle is why MIT Officer Sean Collier was killed in cold blood.
One of the Tsarnaev brothers reportedly approached the officer's car window and shot him at close range. No one knows why. The ambush was the first event that set off a massive dragnet on the night of April 18.
quicklist: 4title: How did Tamerlan Tsarnaev die?text:Tamerlan Tsarnaev, 26, the elder of the suspected Boston Marathon bombing brothers, died following a shootout with authorities Thursday night.
Following the shootout, the wounded Tsarnaev was taken to Beth Israel Deaconess Medical Center. He was pronounced dead there at 1:35 a.m. on April 19. The hospital initially said he died of blast wounds and other wounds too numerous to count.
But Watertown Police Chief Edward Deveau told the Boston Globe this weekend that the elder Tsarnaev died when his brother Dzhokhar Tsarnaev ran over him in a car during his attempt to get away. The body was dragged on the pavement, causing the fatal injuries, the chief said.
A cause of death for Tamerlan Tsarnaev has not yet been determined, according to Terrel Harris of the state Executive Office of Public Safety and Security.
"The medical examiner is waiting [to] review additional information before making the determination," Harris told ABCNews.com in an email today.
quicklist: 5title: What happened in the showdown with authorities when Dzhokhar Tsarnaev was hiding in the boat?text:
When a Watertown resident alerted authorities that he had found a bleeding man in his boat in his backyard, authorities swarmed to the scene.
According to police, a helicopter with infrared technology located Tsarnaev in the boat and noted that he was moving. The helicopter directed officers on the ground to the boat, where they briefly exchanged gunfire shortly before 7 p.m.
"It was a long period. There was probably a 10, 15 minute period where they are trying to get [Tsarnaev] to pay attention. So negotiations went on. There was no conversation back and forth. At the end they were just making demands of him: Show your hands, lift your shirt. And eventually that's what he did," Deveau told ABC News.
Conflicting reports have emerged about how Dzhokhar got out of the boat, some saying he fell out and others saying authorities scooped the weakened man out.
"He was very slow and lethargic in every move that he made and they could see that there was no device on his chest. They kept creeping closer to him and then they felt it safe enough to pull him away from the boat," he said.
quicklist: 6title: How did Dzhokhar Tsarnaev get the neck wound?text:Dzhokhar Tsarnaev suffered a neck wound and has not been verbally communicating with investigators so far, according to authorities. He has been communicating sporadically in writing.
Deveau believes the wound to Tsarnaev's throat occurred while he was in the boat, but he can't be certain at this point. It is unknown at this time how he got the wound. Some have speculated that it could be self-inflicted.
Tsarnaev suffered gunshot wounds to the head, neck, legs and hand, but it is unknown if these were from the first shootout, the boat gunfire or both.
quicklist: 7title: Why didn't the Tsarnaevs flee after the bombing?text:Another puzzling issue is the question of why the suspects did not flee the city, state or even country after the attack.
Deveau told ABC News believes the brothers stayed in the area because they wanted to do more damage.
"These guys did the Boston Marathon bombing, they did not flee," Deveau told ABC News. "They had their pictures put into the media all over the world. They did not flee. They still had bombs...They had a plan to kill more people."
Following the attack, Dzhokhar was back on campus at the University of Massachusetts Dartmouth and even appeared to tweet about being stress free.
Contributing reporting by ABC News' Martha Raddatz, Kirit Radia and Pierre Thomas.