Jurors in the trial of Boston Marathon bomber Dzhokhar Tsarnaev heard a dramatic recording of a Massachusetts Institute of Technology police officer frantically calling for help after discovering an officer who'd been gunned down.

MIT Officer David Sacco Wednesday described receiving a call from a resident who heard loud noises and saw an MIT cruiser in a courtyard on the university's campus in Cambridge on April 18, 2013, three days after the bombings. He said he was unable to reach Officer Sean Collier and 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 and near death. "Get on it!" the officer yells.

Nathan Harman, 24, a student working toward a PhD in mathematics, said he was riding his bike through the courtyard at about 10:20 p.m. that day, when he saw a parked MIT police cruiser. Harman said the driver's-side door was open and there was a man bent at the waist and leaning in to the cruiser.

"He sort of snapped up, stood up and turned around, and he looked startled," Harman said. "And then I just didn't think anything of it and rode off."

When asked by a prosecutor if he saw the person in the courtroom, Harman pointed to Tsarnaev. "He's right there," he said.

Harman said he assumed Tsarnaev was an MIT student.

"I remember thinking he had a big nose, but nothing beyond that really," he said.

Prosecutors say Tsarnaev and his older brother, Tamerlan, killed Collier in a failed attempt to steal his gun. The shooting came hours after the FBI released images of the two brothers as suspects in the marathon bombing.

Tsarnaev's lawyers have admitted he participated in the bombing, but say Tamerlan was the mastermind. They also say it was Tamerlan who shot Collier while trying to steal his gun.

Three people were killed and more than 260 injured when the brothers set off two pressure-cooker bombs near the marathon finish line. Dzhokhar, now 21, faces the possibility of the death penalty if convicted.

Testimony was set to resume Thursday.