Jurors in the Boston Marathon bombing trial Wednesday heard a frantic call made by a police officer who discovered Massachusetts Institute of Technology officer Sean Collier who was shot in his cruiser three days after the bombing.

Prosecutors say Dzhokhar Tsarnaev and his brother, Tamerlan, killed Collier in a failed attempt to steal his gun as they tried to flee after the FBI released photos of them as suspects in the bombing.

MIT Officer David Sacco described receiving a call from a resident who heard loud noises and saw an MIT police cruiser outside. After being unable to reach Collier, Sacco sent another officer to check on him.

The jury heard a recording of that officer yelling "Officer down! Officer down!" when he discovered Collier shot multiple times.

Dzhokhar Tsarnaev's lawyer told the jury Tamerlan shot Collier.

The Boston Herald reported that the chief of MIT police department, John DiFava, testified that he told Officer Collier to "be safe" about an hour before prosecutors say he was killed by the Boston Marathon bombers.

Jurors were also shown the tattered pieces of the backpack that Tsarnaev used to carry a bomb in the deadly 2013 attack.

FBI Special Agent Sarah De Lair held up pieces of the black and white backpack for the jury. De Lair said the pieces were found near the Forum restaurant, where Tsarnaev planted and detonated a bomb April 15, 2013.

Three people were killed and more than 260 injured when Tsarnaev and his older brother set off two pressure cooker bombs near the marathon finish line.

Photos show debris and abandoned items scattered on the sidewalk and street — clothing, a stroller, plastic water bottles and matching cowboy boots a few feet apart.

Dzhokhar Tsarnaev's lawyers have admitted he participated in the bombing but say Tamerlan was the mastermind. Tamerlan, 26, died in a shootout with police four days after the bombing.

Tsarnaev, now 21, faces the possibility of the death penalty if convicted.

The Associated Press contributed to this report