The blood of a Massachusetts Institute of Technology police officer appeared on white gloves found in Boston Marathon bomber Dzhokhar Tsarnaev's car, a DNA expert testified Wednesday as prosecutors tried to show he played an active role in that killing days after the bombings.

Tsarnaev, 21, faces the possibility of the death penalty for his role in the 2013 deadly bombings. Three people were killed and more than 260 were injured when two pressure-cooker bombs exploded near the finish line of the marathon.

Tsarnaev's lawyer admitted during opening statements that he participated in the bombings but said his older brother, Tamerlan, was the mastermind. His lawyer also said it was Tamerlan, not Dzhokhar, who shot Officer Sean Collier as the brothers tried to flee.

Jennifer Montgomery, a DNA analyst with the state police crime lab, testified Wednesday that Collier's blood was on a pair of gloves found near the gas pedal and driver's seat of Tsarnaev's Honda Civic. Prosecutors have said the brothers fled the scene of Collier's shooting in the Honda.

An MIT graduate student who was riding his bike by the scene of the shooting identified Dzhokhar Tsarnaev as the man he saw leaning into Collier's police cruiser the night he was killed. Collier was shot six times, including three times in the head at close range.

During cross-examination by Tsarnaev's lawyers, Montgomery said she also tested blood on the sweatshirt Dzhokhar wore when Collier was shot. None of the blood was Collier's; it was all Dzhokhar's blood, she said.

After Collier's killing, the Tsarnaevs had a wild shootout with police in Watertown. Tamerlan Tsarnaev died following the gunfight and being run over by his brother as he fled. Dzhokhar was found hours later, wounded and bloodied, hiding in a boat parked in a backyard in Watertown.

During earlier testimony Wednesday, jurors were shown two pipe bombs hurled at police during the shootout. Neither of those bombs exploded, but a third pipe bomb did. A pressure-cooker bomb similar to the bombs used in the marathon attacks caused a powerful explosion that shook houses on the street and sent debris raining down on police. Trooper Robert McCarthy also showed the jury a Tupperware container filled with explosive powder and fuses found in a car stolen by the Tsarnaevs just before the firefight with police.

Tsarnaev's lawyers showed the jury a photo of receipts Tamerlan had from a gun and ammunition store and for two backpacks and a soldering gun purchased in the days and weeks before the marathon bombings, apparently in an attempt to show that Tamerlan had a larger role in the bombings than Dzhokhar did.

While cross-examining a fingerprint expert, Tsarnaev's lawyers asked him to identify receipts found inside Tamerlan's wallet. Also in the wallet was a MoneyGram receipt reflecting a $900 transfer to Russia by Tamerlan.

Testimony is scheduled to resume Thursday.