Officer Matthew Harrity says he had a "weird feeling" and heard a thump on his police vehicle before partner Mohamed Noor opened fire on Justine Damond Ruszczyk.
Minneapolis police officer Matthew Harrity has testified he feared a potential ambush just before his partner Mohamed Noor shot dead Australian life coach Justine Damond Ruszczyk in an alley.
Officer Harrity's testimony could prove to be powerful as it echoes Noor's argument he was acting in self-defence when he fired the fatal bullet.
Noor has been charged with second-degree murder, third-degree murder and manslaughter for the fatal shooting that shocked Minneapolis and Australia.
Noor's murder trial in a downtown Minneapolis courtroom is in its second week.
Officer Harrity described on Thursday how he felt a "weird feeling" and drew his gun after he heard a "murmur" and "a thump" on his stationary police vehicle that was located at the end of an alley behind Ms Damond Ruszczyk's Minneapolis home late on the evening of 15 July in 2017.
"In this situation, with the thump and being startled, I went straight to, 'this could be an ambush'," Harrity said.
"My first thought is, I'm going to make sure whatever it was is not a threat to me," he added.
Ms Damond Ruszczyk called 911 after hearing a woman's screams and fearing a rape was underway near the alley behind her Minneapolis home.
Officer Harrity drove a police Ford Explorer with Noor in the front passenger seat.
Officer Harrity said as he tried to make sense of the thump and noises, he heard a pop and looked over to see that Noor, from the passenger seat, had fired across him and through the window.
Ms Damond Ruszczyk, barefoot and dressed in her pyjamas, had approached the squad car and was hit in the stomach.
A medical examiner testified earlier the bullet struck a key artery and Ms Damond Ruszczyk lost so much blood so quickly that even faster medical care might not have saved her.
Neither officer had their body cameras running at that point, something Officer Harrity blamed on what he called a vague policy that didn't require it.
Both men switched them on afterwards, and a portion of Officer Harrity's was played on Thursday.
It showed efforts by the two men to save Ms Damond Ruszczyk with CPR.
Ms Damond Ruszczyk's laboured breathing is heard, with Officer Harrity saying, "stay with me, stay with me, stay breathing".
He also is heard addressing his partner: "Noor, breathe, just breathe."
At one point, as Officer Harrity stepped away to get medical supplies, he cautioned Noor to slow down the CPR and reassured Noor that an ambulance was coming.
It is not clear whether Noor will testify.
Noor, 33, is a Somali American whose hire two years before the shooting was celebrated by Minneapolis leaders as a sign of a diversifying police force in a city with a large population of Somali immigrants.
Much of the prosecution's early case focused on the handling of the crime scene by police and state Bureau of Criminal Apprehension agents, including possible missteps.
They also highlighted officers turning their body cameras on and off repeatedly after the shooting.