Justine Damond: Police startled by ‘loud sound’ before fatally shooting Australian bride-to-be

US police were startled by a "loud sound" seconds before an officer shot and killed an Australian woman outside her home.

Justine Damond, 40, had called 911 to report an assault in an alley behind her house in Minneapolis, Minnesota, when she was shot by officer Mohamed Noor as she approached the driver's side window of his police car.

The account of the killing comes from officer Matthew Harrity, who was driving the car. Mr Noor has so far refused to be interviewed. They are both on paid administrative leave.

Yesterday, audio emerged of the moments surrounding the fatal shooting, in which the officers can be heard calling for backup and attempting to perform CPR on Ms Damond.

According to the Minnesota Bureau of Criminal Apprehension, Ms Damond approached the driver's side window of the police car immediately after Mr Harrity said he had been startled by the sound.

Mr Noor, in the passenger seat, then shot her through the open driver's side window. No weapon was found at the scene.

Mr Harrity told investigators he and his partner responded to a 911 call from Ms Damond about a possible assault near her home around 11:30pm on Saturday.

Mr Harrity was driving the car as the officers went through an alley to look for a suspect. The squad lights were off when the noise startled him, Mr Harrity said.

The officers did not turn on their body cameras until after the shooting, and the police car's camera was also not activated.

Mr Harrity told investigators that after the shooting, the officers got out of their car and gave Ms Damond immediate medical attention.

He said he and Mr Noor saw a man, thought to be between 18 and 25, bicycling in the area before the shooting. The man stopped and watched as officers attended to Ms Damond.

BCA agents are asking that man, and any other potential witnesses, to come forward.

The BCA said unless more people come forward, there are no additional interviews scheduled.

David Klinger, a criminal justice professor at the University of Missouri-St Louis, said police officers can't be compelled to testify in an outside investigation.

"Police officers are citizens ... they have the same Fifth Amendment right as anyone. They don't have to give a statement," Mr Klinger told the Press Association. "His lawyer might be saying, you're not going to talk until I feel you're rested and not under stress."

In a news conference after the BCA's update, Mayor Betsy Hodges said she wished Mr Noor would speak to investigators.

"It's frustrating to have some of the picture but not all of it," she said. "We cannot compel Officer Noor to make a statement. I wish we could. I wish that he would make a statement."

Malcolm Turnbull, the Australian Prime Minister, gave an interview to Australia's Today show shortly before the release of details from the BCA's preliminary investigation, and joined the chorus demanding answers.

"How can a woman out in the street in her pyjamas seeking assistance from police be shot like that? It is a shocking killing," Mr Turnbull said.

In Ms Damond's hometown of Sydney, about 300 people attended a silent vigil in her honour on Wednesday morning at Freshwater Beach. Mourners threw pink flowers into the Pacific Ocean.

Records from the city's Office of Police Conduct Review show Mr Noor has had three complaints against him. Two are pending, and the third was dismissed without discipline. Under state law, details of open cases and cases that result in no discipline are not released.

Mr Noor was also sued earlier this year after a 25 May incident in which he and other officers took a woman to the hospital for an apparent mental health crisis.

The lawsuit claims Mr Noor and other officers violated the woman's rights when they entered her home without permission and he grabbed her wrist and upper arm. The lawsuit, which is pending, said Mr Noor relaxed his grip when the woman said she had a previous shoulder injury.

Ms Damond, who was planning to be married next month, was a meditation teacher and life coach. Her maiden name was Justine Ruszczyk, and though she was not yet married, she had already been using her fiance's last name.

Additional reporting by AP