By Elizabeth Barber

BOSTON (Reuters) - A Massachusetts Institute of Technology police officer who found a colleague in his squad car covered in blood three nights after the Boston Marathon bombing frantically repeated two words into his radio "officer down, officer down."

A recording of that radio call was played to jurors hearing the trial of accused bomber Dzhokhar Tsarnaev, 21, on Wednesday, as prosecutors turned to the charge that the defendant and his older brother murdered MIT police officer Sean Collier on April 18, 2013, in an unsuccessful attempt to steal his gun.

Collier's death marked the start of a chaotic 24 hours that saw the brothers carjack a man and hurl explosives at police during a shootout that ended when Dzhokhar Tsarnaev roared off in a car, running over and killing 26-year-old Tamerlan Tsarnaev before disappearing into a drydocked boat in the Boston suburb of Watertown. Police found him the next evening, after a day-long lockdown of much of the Boston area when hundreds of thousands of people hid in their homes.

The officer who found Collier, Sergeant Clarence Henniger, said he was responding to a 911 call about possible shots fired on MIT's Cambridge, Massachusetts, campus, just outside Boston. Collier had visible bullet wounds to his temple, neck and hand and was so covered in blood that Henniger struggled to pull him from his vehicle.

"I could hear gargling coming from his mouth," Henniger testified at U.S. District Court in Boston, where the jury hearing Tsarnaev's trial will also decide whether to sentence him to death if he is convicted of killing three people and injuring 264 with a pair of homemade bombs at the April 15, 2013, race.

Jurors also saw surveillance footage of two figures approaching Collier's police cruiser on the MIT campus, spending about a minute at the car, and leaving. The only visible suggestions of commotion in the video are the car’s brake lights, flashing on and off.

Collier's gun belt and holster were covered in blood, evidence of Tsarnaev's clumsy but unsuccessful effort to steal his handgun, prosecutors have said.

Tsarnaev's attorneys opened the trial by admitting he committed the crimes of which he is accused, but are seeking to spare him the death penalty by demonstrating he was following the lead of his older brother.

Federal prosecutors contend Tsarnaev, who emigrated with his family from Chechnya a decade before the attack, was driven by an extremist view of Islam and a desire to take revenge on the United States for military campaigns in Muslim-dominated countries.

Prosecutor William Weinreb said in his opening statement last week that it was unclear if Tsarnaev or his brother fired the weapon that killed Collier but argued that both brothers were equally culpable in the officer's death.

A graduate student at MIT, Nathan Harman, who was bicycling on campus the night of the murder, testified on Wednesday that he saw Tsarnaev leaning into the driver's side door of Collier’s car, but did not see the older sibling.

"I only saw the one person," said Harman.

MIT Police Chief John DiFava on Wednesday recalled chatting with Collier about an hour before his death.

"I told him to be safe, and I left," DiFava said, adding it was the last time he saw Collier, 27, alive.

(Editing by Scott Malone and Tom Brown)