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 Noorshot 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.
Officer Harrity was served a subpoena on Wednesday, local time, 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."
Justine Damond's parents, John Ruszczyk and Maryan Heffernan, are seen embracing at the water's edge during a vigil for their daughter at Freshwater Beach on July 19, 2017 in Sydney. Photo / Getty
Freeman has been under intense public pressure to prosecute Officer 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 co-operate 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."
Officer Noor has refused to speak to investigators.
Officer Harrity's testimony will be key as he was in the car with Officer Noor when the shot was fired.
Freeman has said Officer Harrity told investigators little about the shooting.
Damond, 40, dressed in her pyjamas, was shot by Officer Noor after she called 911 just before midnight on July 15 to report a woman screaming in her affluent Minneapolis neighbourhood.
Police officer Mohamed Noor pictured in May 2016 was the officer who shot and killed Justine Damond. He has refused to speak about the events leading up to the shooting. Photo / AP
Freeman has previously expressed his frustration over delays in the investigation. During a union holiday in December, he told activists that he didn't have enough evidence to charge Noor and said investigators "haven't done their job".
He also suggested Noor's refusal to speak to investigators 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," Freeman said.
"But I can't. He won't answer my questions because he doesn't have to, OK?"
Freeman later apologised to investigators and said his comments were ill-advised.
Minneapolis Mayor Jacob Frey said in a statement: "Arriving at the right decision requires the right facts and complete truth. No institution – including the City of Minneapolis – should stand in the way of uncovering that truth."
