Sydney native Justine Damond had called police to report a suspected sexual assault
US police were startled by a "loud sound" moments before an officer shot dead an Australian woman near her home, investigators say.
Justine Damond, 40, called police to report a suspected sexual assault and was standing outside the driver's door of a squad car in Minneapolis, Minnesota when she was shot dead by officer Mohamed Noor.
The driver, officer Matthew Harrity, told investigators that he had been "startled by a loud sound" near the car just before Ms Damond approached, the Minnesota Bureau of Criminal Apprehension (BCA) said.
The sound was not identified but an unidentified officer recorded on police dispatch radio said it may have been fireworks that sounded like gunshots.
Mohamed Noor refused to be interviewed by investigators and his lawyer Tom Plunkett "did not provide clarification on when, if ever, an interview would be possible," the BCA added.
In a statement, Mr Plunkett said the officer was "a caring person" who "empathises with the loss others are experiencing."
Minneapolis mayor Betsy Hodges said authorities could not "compel" Noor to speak about the incident, but said he should as "he has a story to tell that no one else can tell."
She added: "It's frustrating to have some of the picture but not all of it."
At the time of the shooting, the two officers' body cams were not switched on and footage from the police car failed to capture the incident.
Australian Prime Minister Malcolm Turnbull described the death as "inexplicable" and said his government was trying to find out what "went tragically wrong."
He said: "How can a woman out in the street in her pyjamas seeking assistance be shot like that?”
"It is a shocking killing, and yes, we are demanding answers on behalf of her family."
Ms Damond, a meditation teacher from Sydney, died from a gunshot wound to the abdomen.
The 40-year-old was due to marry fiancé Don Damond, whose name she had already taken, in August.
Ms Damond's family in Australia said they were "trying to come to terms with this tragedy and to understand why this has happened."
On Wednesday, hundreds of family members and friends gathered for a vigil on Sydney's Freshwater beach, where they stood in silence before casting pink flowers into the water.