The family of Justine Damond has welcomed a secret grand jury to probe key witnesses under oath about why a police officer shot dead the Australian life coach.

The move to convene the jury by the top prosecutor in the US city of Minneapolis comes after the case appeared to hit a wall just before Christmas.

Ms Damond, 40, was gunned down in her pyjamas by Somali-American officer Mohamed Noor on July 15 last year after calling 911 to report a woman screaming.

A secret grand jury will be convened to probe key witnesses under oath about why a police officer shot dead Australian life coach Justine Damond (pictured)

The move by the top prosecutor in the US city of Minneapolis comes after the case appeared to hit a wall just before Christmas (pictured is Mohamed Noor)

Noor shot across his partner, Officer Matthew Harrity, out of their police car window at Ms Damond as she approached their vehicle in a dark alley behind her home.

Officer 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.

Ms Damond, 40, was gunned down in her pyjamas by Somali-American officer Mohamed Noor on July 15 last year after calling 911 to report a woman screaming (pictured is officer Matthew Harrity)

Noor, 31, has refused to speak to investigators and the chief prosecutor, Hennepin County Attorney Mike Freeman, is yet to charge him over the killing.

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

'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,' Mr 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.'

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

Noor, 31, has refused to speak to investigators and the chief prosecutor, Hennepin County Attorney Mike Freeman (pictured), is yet to charge him over the killing

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

Ms Damond's family welcomed the grand jury, in a statement by the family's lawyer, Bob Bennett.

'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,' and Mr Bennett.

'I question whether [witnesses] have been totally forthcoming or told the truth in whole,' he said in the statement to the Minneapolis Star-Tribune.

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

Mr Freeman has said Officer Harrity told investigators little about the shooting.