Pc Andrew Harper fell “as if his legs were whipped from under him” moments before he was dragged to his death, a court has heard.
The 28-year-old Thames Valley Police officer was carried for more than a mile behind a Seat Toledo after attempting to apprehend quad bike thieves.
The Old Bailey has heard that Pc Harper died from catastrophic injuries after his legs got entangled in a tow rope attached to the back of the Seat, driven by 18-year-old Henry Long.
Giving evidence, Pc Andrew Shaw said his crew mate got out of their patrol car when they spotted the Seat, which had been towing the quad bike.
He said: “I could see him running in the road. My first thought was he’s running after the car trying to get in it. It’s what he would have done.
“I thought he was trying to get to the open door, drag them out of the car.
“As the car accelerated, Pc Harper was standing there and he just appeared to fall back as if his feet had been whipped from under him and that’s pretty much the last I saw of him.
“It was so quick you don’t even have time to put your hands out to steady yourself.”
As he reacted to the fast-paced events, Pc Shaw said his biggest concern was that he did not reverse into his colleague.
A call handler had alerted the officers to a burglary in progress near Stanford Dingley in Berkshire, after 11pm on August 15 last year.
She relayed the report of “four masked men outside with weapons, pieces of wood, trying to break into the garage, trying to steal his quad bike”.
In the radio transmissions played in court, Pc Harper responded: “We are with it, we are with it.”
Pc Shaw is heard saying: “My colleague Pc Harper got out of the vehicle, ran after the vehicle. I have now lost him.”
He is asked to confirm the location, before a communication from another officer, Pc Christopher Bushnell, states: “There is a body in the road, body in the road. Just fell out of the vehicle.”
Pc Shaw replies: “That’s probably Pc Harper. I’ve just found his stab vest in the road.”
The officer eventually found his colleague lying in the road and, with others, tried to save his life before he was pronounced dead at the scene.
Jurors were shown video footage from the dashboard of Pc Shaw’s car as it drove past bloodied streaks in the road.
It ended with a view of Pc Shaw stopping and rushing over to join an officer standing over Pc Harper, who was lying in the road.
Jurors have heard that Pc Harper was probably knocked unconscious in the initial fall and his injuries were not survivable.
Pc Shaw told jurors he had been a member of the Thames Valley force’s road policing proactive team since 2009, having joined up in 1990.
The advanced police driver said he had known Pc Harper, nicknamed Harps, for about 18 months, adding that he had joined the team six weeks earlier.
On August 15 last year, they had been due to finish their shift at 7pm but, due to a surveillance job, were not stood down until 10pm, he said.
Prosecutor Brian Altman QC asked Pc Shaw: “This was well beyond your shift ended. Did that make a difference whether you went to the scene?”
The officer replied: “None at all.”
Long, from Mortimer, Reading, and two 17-year-old boys, who cannot be named for legal reasons, have denied Pc Harper’s murder but admitted conspiracy to steal a quad bike.
Long has also admitted manslaughter, which his co-accused deny.