The police officer who fatally shot Australian woman Justine Ruszczyk in Minneapolis on Saturday reportedly thought he was being caught in an ambush.

Mohamed Noor and his partner Matthew Harrity responded to an alley near the spiritual healer’s home after she dialled 911 to report a woman’s screams.

The officers, both with their body cameras switched off, drove south in search of a suspect with their squad car lights switched off, according to Minnesota's Bureau of Criminal Apprehension (BCA).

"As they reached West 51st Street, Officer Harrity indicated that he was startled by a loud sound near the squad (car),” the BCA said in a statement.

The crowd paid silent tribute to the 40-year-old, before striking a gong. (9NEWS)

People attending the vigil were urged to bring pink flowers to honour Ruszczyk. (9NEWS)

The family said the vigil is a chance to "come together as a community" to mourn their loss. (9NEWS)

The sound was reportedly a ‘banging’ on the window and the officers saw a young man riding past on a bicycle near the scene, local media KSTP reports.

Authorities now believe the cyclist could hold key information that could assist with investigations.

Mr Harrity told the BCA Ruszczyk approached the driver's side window immediately after the sound, and Mr Noor shot her through the open driver's side window.

Mr Noor allegedly had his gun on his lap at the time.

The lawyer for Mr Harrity said it is "certainly reasonable" his client feared he was the target of an ambush.

Lawyer Fred Bruno pointed to the recent execution of a New York police officer gunned down as she sat in her car.

"It's certainly reasonable to assume that any police officer would be concerned about a possible ambush under these circumstances," Mr Bruno told the Minneapolis Star Tribune newspaper.

"It was only a few weeks ago when a female NYPD cop and mother of twins was executed in her car in a very similar scenario."

NYPD Officer Miosotis Familia was shot in the head on July 5 while sitting in her car in the Bronx.

In Sydney, Ruszczyk’s grieving family and friends have cast flowers into the water at a beach to pay tribute to her.

The 40-year-old’s father, John Ruszczyk, was emotional as the crowd walked to the Freshwater Beach shoreline yesterday morning to remember the woman who was raised on the Northern Beaches and attended Manly High School.

Simultaneously, dozens of people from the US, Brazil, Sweden and France posted on a Facebook page promoting the "silent tribute" at Freshwater.

Mr Noor has refused to be interviewed by police, a decision Ruszczyk’s family has said is “keeping them in the dark.”

"We cannot compel officer Noor to make a statement," Minneapolis Mayor Betsy Hodges said.

“I wish he would,” she added.

Meantime Mr Noor's family have declared his innocence, saying it was a "mistake".

“We feel so bad about this, we are traumatised ourselves. It’s so unfortunate,” a family member said.

“If you wait for the investigation you’ll know it was an honest and sincere event that transpired. Until then we can’t really say anything.”

Australian Prime Minster Malcolm Turnbull told 9NEWS he wanted answers.

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

“It is a shocking killing."