Jurors in Boston are considering whether Dzhokhar Tsarnaev is guilty of carrying out the bombing of the 2013 city marathon.
The jury of seven women and five men began the process on Tuesday, nearly two years after the attacks killed three and injured more than 260 people.
The court heard that Mr Tsarnaev “wanted to punish America” for its wars in Muslim countries.
Defence lawyers concede Mr Tsarnaev played a role but want his life spared.
They say he should not be executed because his older brother Tamerlan, killed in the aftermath, was the mastermind.
“If not for Tamerlan, it would not have happened,” Judy Clarke said, during arguments in which she repeatedly referred to her client as a “kid” and a “teenager”.
But the prosecution presented a man who killed without a thought for the innocents.
“This was a cold, calculated terrorist act. This was intentional. It was bloodthirsty. It was to make a point,” Aloke Chakravarty said.
If Mr Tsarnaev is found guilty, the same jury will then begin hearing evidence on whether he should face life imprisonment or execution as part of the second phase of the trial.