Friends and family are demanding answers as to why US police shot dead a 40-year-old Australian woman.

The victim has been identified as Justine Damond, also known as Justin Ruszczyk, who was living in Minneapolis with her fiancé.

A devastated community are now trying to figure out how she was shot down after calling police to report an alley way assault.

Her 22-year-old step-son whose father was engaged to be married to the the Australian in August, said she was shot "for no reason".

"Basically my mom's dead because a police officer shot her for reasons I don't know," he said in a heartbreaking video moments after the shooting.

View photos Justine Damond was shot dead after reporting an alley way assault to police. Source: 7 News More

View photos The community rallied together for a vigil at the scene of the crime, demanding answers at how the much loved Australian woman was killed. Source: Facebook More

"I demand answers. If anybody can help, just call police and demand answers," he added.

"I'm so done with all this violence, America sucks."

Police bodycameras were switched off, while the dashcam was reportedly not working at the time of the shooting.

Two Minneapolis police officers responded to a 911 call of a possible assault just north of the 5100 block of Washburn Avenue South just before 11.30pm Saturday, Minneapolis police confirmed in a statement.

View photos Neighbours are now demanding answers, leaving a sign out asking police More

View photos Devastated friends and family gathered on the suburban street. Source: Twitter More

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