A Minneapolis police officer who shot and killed unarmed Australian woman Justine Damond last July has turned himself in on charges of third-degree murder and second-degree manslaughter.

Key points: Third-degree murder carries a maximum 25 years in prison

Third-degree murder carries a maximum 25 years in prison Justine Damond's family releases statement saying they hope for conviction

Justine Damond's family releases statement saying they hope for conviction Mohamed Noor has not spoken publicly about the case

Officer Mohamed Noor shot Ms Damond, a 40-year-old life coach, on July 15, minutes after she called 911 to report a possible sexual assault in the alley behind her home.

Her death drew international attention, cost police chief Janee Harteau her job and forced major revisions to the department's policy on body cameras.

Hennepin County Attorney Mike Freeman outlined the case against Mr Noor in a media conference this morning (AEDT).

"We have a second-by-second understanding of what happened," Mr Freeman said.

Sorry, this video has expired Hennepin County Attorney Mike Freeman outlines the case against Mohamed Noor

"In the short time between when Ms Damond Ruszczyk approached the squad car and the time that officer Noor fired the fatal shot, there is no evidence that officer Noor encountered a threat, appreciated a threat, investigated a threat or confirmed a threat that justified his decision to use deadly force.

Minneapolis police officer Mohamed Noor has not spoken publicly about the case. (Supplied: Hennepin County Sheriff's Office)

"Instead officer Noor intentionally and recklessly fired his handgun from the passenger seat in disregard for human life."

Mr Noor has been booked on a third-degree murder charge for perpetrating an eminently dangerous act while showing a "depraved mind".

The second-degree manslaughter charge alleges he acted with "culpable negligence creating unreasonable risk".

If convicted of third-degree murder, he could face a maximum of 25 years in prison, though the presumptive sentence is 12 and a half years.

His lawyer, Thomas Plunkett, confirmed Mr Noor turned himself in, but had no other immediate comment.

The second-degree manslaughter charge carries a maximum sentence of 10 years in prison, but the presumptive sentence is four years.

Mr Noor has not spoken publicly about the case and declined to answer questions from investigators.

Ms Damond's father, John Ruszcyzk, and her fiance, Don Damond, issued a joint statement on behalf of both families, saying they applauded the decision to charge Mr Noor "as one step toward justice for this iniquitous act".

They said they were pleased that the investigation appeared diligent and thorough, and they hoped for a conviction.

"No charges can bring our Justine back," the statement said.

"However, justice demands accountability for those responsible for recklessly killing the fellow citizens they are sworn to protect, and today's actions reflect that."

A policeman who was with Mr Noor at the time of the shooting, Matthew Harrity, told investigators that he was startled by a loud noise right before Ms Damond approached the driver's side window of their police car.

Mr Harrity, who was driving, said Mr Noor then fired his weapon from the passenger seat.

Ms Damond died of a gunshot wound to the abdomen.

Watch Duration: 2 minutes 17 seconds 2 m 17 s Justine Damond's loved ones open up to Australian Story about the battle for justice

Lack of video evidence criticised

The officers did not turn on their body cameras until after the shooting, and there was no squad camera video of the incident.

The lack of video was widely criticised, and Ms Damond's family members were among the many people who called for changes in procedure, including how often officers are required to turn on their cameras.

The shooting also prompted questions about the training of Mr Noor, a two-year veteran and Somali-American whose arrival on the force had been celebrated by city leaders and Minnesota's large Somali community.

Mr Noor, 32, had trained in business and economics and worked in property management before becoming an officer.

Then-chief Ms Harteau defended Mr Noor's training and said he was suited to be on the street, even as she criticised the shooting itself.

But Ms Harteau — who was on holidays when the shooting happened and did not make her first public appearance until several days after the shooting — was forced out soon after by Mayor Betsy Hodges, who said she had lost confidence in the chief.

Ms Harteau's replacement, Medaria Arradondo, quickly announced a policy change requiring officers to turn on their body cameras in responding to any call or traffic stop.

Mr Noor has been on paid leave since the shooting.

AP