"They did it to protect their close friend," said Assistant U.S. Attorney Stephanie Siegmann.
Tazhayakov is charged with obstructing justice and conspiracy to obstruct justice in connection with alleged actions taken after the bombing. His roommate, Dias Kadyrbayev, faces the same charges, which carry a maximum sentence of 20 years. Both have pleaded not guilty
. Another friend, Robel Phillipos, pleaded not guilty to making false statements. None is accused in the bomb plot itself.
Lawyers for Tazhayakov, a University of Massachusetts Dartmouth student from Kyrgyzstan, said he did everything he could to help the investigation when he spoke with the FBI. Based on that information, authorities found Tsarnaev's backpack in a landfill and his laptop in the students' apartment.
The government says accused bomber Tsarnaev texted Tazhayakov after the bombing and told him he could go to his dorm room and take what they wanted. He ended the text with a smiley face.
His lawyer told jurors, the smiley face is code for marijuana.
Tazhayakov asked whether Tsarnaev saw the news of the bombing, according to prosecutors.
Tsarnaev responded, "Don't go thinking it's me, you cooked bastard," which reportedly is a phrase used to describe those who smoke marijuana. A friend who took the stand as one of the first prosecution witnesses told jurors Tsarnaev sold weed on campus.
Prosecutors told jurors Tazhayakov knew the identity of the suspected bombers -- Tsarnaev and his older brother Tamerlan Tsarnaev -- before the public found out, allegedly texting Kadyrbayev, "i think they got his brother" hours before the public knew their names or their relationship to one another. That's because the students allegedly recognized the Tsarnaev brothers after authorities released video and still photos asking for the public's helping finding the two men.
Defense lawyers for Tazhayakov and Kadyrbayev have said the young men weren't sure it was the Tsarnaev brothers. Kadyrbayev will have a separate trial at a later date.
In January 2013, three months before the bombing, the government saia Tazhayakov stayed over with his pal for two nights and met Tamerlan Tsarnaev, who died in a shootout with police days after the bombing. The FBI suspects Tamerlan helped plot and carry out the terror attack with his little brother.
One month before the bombing, the government says the younger Tsarnaev met with his friends at a restaurant and told them he knew how to make a bomb and could make one with gunpowder. He also allegedly discussed martyrdom.
"It was good to die ... as a martyr, as you would die with a smile on your face and go straight to heaven," Tsarnaev told them, according to Siegmann.
Tazhayakov's lawyer Nicholas Wooldridge said his client doesn't believe in martyrdom and the Tsarnaevs did not act as martyrs because they initially survived the attacks.
The defense maintains Tazhayakov, then 19, didn't understand his legal right to remain silent when the FBI questioned him because of difficulty understanding English and that he was coerced during questioning. His lawyers have asked a judge to throw out his statement, but the judge has not yet ruled on that motion.
His lawyers also blame co-defendant Kadrybayev for throwing the backpack in a dumpster, repeatedly telling jurors Tazhayakov never touched the backpack and didn't know it had been ditched.
"We've been asking God that the truth will come out, and we hope the jury will hear that," Tazhayakov's father said outside court, through a Russian translator.
When CNN asked whether his son could have done something differently, his father replied "Absolutely. Before you make a friend, make sure you make the right decision."