A Minnesota prosecutor has said he does not yet have enough evidence to charge a Minneapolis police officer who killed an unarmed Australian woman earlier this year, blaming investigators who "haven't done their job".

Key points: Hennepin County Attorney Mike Freeman was filmed saying investigators had not done there job

Hennepin County Attorney Mike Freeman was filmed saying investigators had not done there job Mr Freeman is investigating shooting of Justine Damond by police officer Mohamed Noor

Mr Freeman is investigating shooting of Justine Damond by police officer Mohamed Noor Mr Freeman expected to make a charging decision by the end of the year

Hennepin County attorney Mike Freeman is still deciding whether to charge officer Mohamed Noor, who shot Justine Damond in the alley behind her home in July.

Ms Damond had called 911 to report a possible sexual assault.

As she approached the squad car, Mr Noor fired from the passenger seat, across his partner and through the driver's window.

Ms Damond's death sparked protests and led to a police department shake-up, including the resignation of Chief Janee Harteau.

Mohamed Noor shot Justine Damond as he sat in the passenger seat of a Minneapolis police car ( AP: City of Minneapolis )

Mr Freeman was captured on video expressing his frustration at a Minneapolis Regional Labor Federation holiday reception on Wednesday night, after he was asked about a charging decision.

Union member Sam Sanchez recorded the conversation and posted it on Facebook.

It is not clear if Mr Freeman knew he was being recorded.

In the video, Mr Freeman said he does not have enough evidence yet to decide whether to charge Mr Noor, who has declined to speak with investigators.

"I've got to have the evidence. And I don't have it yet. And let me just say, it's not my fault," Mr Freeman said.

"So if it isn't my fault, who didn't do their jobs? Investigators, and they don't work for me. And they haven't done their job."

Freeman didn't name the investigators or their agency, but the state Bureau of Criminal Apprehension is leading the investigation.

The BCA turned the case over to Mr Freeman in September.

The bureau issued a statement on Thursday saying it continued to work with Mr Freeman's office.

"The BCA conducts the majority of officer-involved shooting investigations in Minnesota, and the collaboration between prosecutors and investigators as a case file is reviewed under the statutes is a typical part of the review process," the BCA statement said.

"State law prohibits us from providing additional details, because it is an active investigation."

Mr Freeman had previously said he expected to make a charging decision by the end of the year.

His office acknowledged the video released on Facebook and did not dispute its authenticity.

"We are working diligently on the case to complete the investigation as soon as possible," his office said in a statement.

"Beyond that, we cannot comment at this time."

Sorry, this video has expired Justine Damond's fiance speaks after she was fatally shot by Minneapolis police

Officer refuses to speak about shooting

Mr Freeman indicated that Mr Noor's refusal to speak had put prosecutors in a difficult position.

"I have to prove beyond a reasonable doubt [that] the moment he shot the gun, he feared for his life. And he used force because he thought he was gonna be killed," Mr Freeman said.

"But I can't. He won't answer my questions because he doesn't have to, OK?

"We all have Fifth Amendment rights, and I respect that.

"So I can't talk to her because she's gone, and the other cop just gave us some (expletive), OK? So guess what? I gotta figure out angles of the shot, gun residues, reckless use of force experts."

Mr Sanchez is also an organiser with the Twin Cities Coalition for Justice 4 Jamar, which was formed after fatal shooting of Jamar Clark by a Minneapolis police officer in 2015.

Mr Freeman decided not to charge the officers involved in that case.

Mr Sanchez told Minnesota Public Radio that he was surprised by Mr Freeman's honesty, but disheartened that he hadn't yet made a decision.

Minneapolis laywer Bob Bennett, who represents Ms Damond's relatives in Australia, told MPR he was concerned but not surprised by Mr Freeman's comments.

"I hope that the BCA hasn't so irretrievably damaged the evidence, or failed to recover evidence that should be reasonably expected to be recovered at the time that the crime occurred," Mr Bennett said.

"And I use the term crime pointedly and intentionally."

Mr Noor's attorney, Thomas Plunkett, told MPR he was concerned by Mr Freeman's remark that having enough evidence to make a charging decision would be "the big present I'd like to see under the Christmas tree".

He also said the job of investigators was to gather evidence, not create it.

"No lawyer wants their client placed under a Christmas tree as a present to a vocal segment of the community. That said, this case is about an officer that follows procedure and training," Mr Plunkett said.

"This led to the death of a very fine person which is a horrible tragedy, but not a crime."

AP