LOS ANGELES: A secret grand jury will be convened in Minneapolis to probe key witnesses under oath about why a police officer shot dead Australian life coach Justine Damond.

The move by the US city's top prosecutor comes after the case appeared to hit a wall just before Christmas.

Officer Mohamed Noor shot across his partner, Officer Matthew Harrity, in their police car and out a window at Damond as she approached the vehicle in a dark alley behind her home on July 15 last year.

Harrity was served a subpoena on Wednesday to testify before the grand jury.

The public and media members are blocked from attending grand jury proceedings.

Testimony is used to build a prosecutor's case.

"It has been reported that several witnesses have been subpoenaed before a grand jury as part of the investigation into the officer-involved shooting of Justine Damond Ruszcyzk," Hennepin County Attorney Mike Freeman said in a statement.

"Because grand jury proceedings are secret, we cannot comment on grand jury subpoenas or any testimony that occurs before a grand jury."

Freeman has been under intense public pressure to prosecute Noor, with multiple street protests and rallies held in Minneapolis.

A rally against police violence will take place at the February 4 Super Bowl held in Minneapolis.

Freeman had planned to announce before the end of 2017 whether Officer Noor would face charges for the shooting, but he complained investigators did not do their job and gather enough evidence.

Damond's family welcomed the grand jury.

"I and the family are happy that the Hennepin County Attorney is using every means at their disposal to get people to cooperate in this investigation and suffer the penalties of perjury if they lie to the grand jury," the Damond family's lawyer, Bob Bennett, told the Minneapolis Star-Tribune.

"I question whether [witnesses] have been totally forthcoming or told the truth in whole."

Noor has refused to speak to investigators.

Harrity's testimony will be key as he was in the car with Noor when the shot was fired.

Freeman has said Harrity told investigators little about the shooting.

Damond, 40, dressed in her pyjamas, was shot by Noor after she called 911 just before midnight on July 15 to report a woman screaming in her affluent Minneapolis neighbourhood.

AAP