A passing cyclist could hold the key to investigation into Justine Damond's death.

A mystery cyclist filmed part of the fatal encounter between US police and Australian Justine Damond, offering a potential breakthrough in the investigation.

Minneapolis police officer Mohamed Noor and his partner Matthew Harrity were criticised for not having their body or vehicle dash cameras on when they were called to an alley behind Damond’s home a week ago.

Authorities announced on Friday that they identified and interviewed a male cyclist who was spotted at the scene. Days earlier they had asked for the mystery man to come forward.

The Minneapolis Star Tribune reported on Saturday that a source with direct knowledge of the investigation said the witness filmed part of the encounter but what he filmed has not been released.

Damond, 40, formerly of Sydney’s northern beaches, called police about 11.30 pm on July 15 after hearing a woman’s screams and fearing a sexual assault was taking place in the alleyway outside her home.

The yoga and meditation instructor’s fiance, Don Damond, also spoke to the Star Tribune about his last moments on the phone with her before the police arrived.

“I have played this over in my head over and over,” he said. “Why didn’t I stay on the phone with her?”

He told her to “stay put” and wait for the police, and to “call him back” after they’d arrived.

As Damond approached the responding police vehicle, Officer Noor shot across his partner and struck her in the stomach. She died at the scene.

The ordeal has caused great controversy in the Minneapolis community, with Chief of Police Janeé Harteau resigning at the request of the city’s mayor, Betsy Hodges, over the weekend.

“I’ve lost confidence in the chief’s ability to lead us further,” Hodges said in a statement released on Saturday. “And from the many conversations I’ve had with people around our city, it is clear that she has lost the confidence of the people of Minneapolis as well.”

There was pressure from the public for Hodges to resign as well. A media conference on Friday evening deteriorated into a protest, with demonstrators calling out “Bye bye, Betsy” and saying they’d been “terrorised enough.”

Signs have also been erected throughout the city, depicting a trigger-happy police officer with the caption, “Warning: Police easily startled.”

The police officer involved in the shooting has so far refused to be interviewed by the Minnesota Department of Public Safety Bureau of Criminal Apprehension, which is investigating the shooting.

Noor reportedly will be compelled to answer the questions of an internal investigation within the force. However, these answers will be applicable to an external or criminal investigation.

– With AAP.