The family of a young father who died after being shot in the back by police has filed a $34 million lawsuit, claiming an "inexplicable" and "merciless" use of excessive force.
California man Oscar Grant, 22, was killed by an officer from the state's transit police named in the lawsuit as Johannes Mehserle following an alleged scuffle on a train shortly after 2am on New Year's Day, the San Jose Mercury News reports.
In a claim lodged on behalf of the family, lawyer John Burris claims police hauled Grant and other passengers off the train at a station in Oakland, near San Francisco.
The claim states Grant obeyed the officers' instructions, before an unnamed "Latino officer" slammed him against a brick wall and kneed him in the face.
Grant then slumped to his knees and put his hands in the air "to demonstrate that he was submitting to the Latino officer's thuggish display of authority", the claim says.
He pleaded to the officer not to use a Taser gun on him by making reference to his four-year-old daughter, but the officer allegedly ploughed his knee into Grant's back causing him to "yell in agony".
The claim states Mehserle, who was watching nearby, stepped in to restrain Grant then "inexplicably" stood up and pointed a gun directly at his back.
"Without so much as flinching, Officer Mehserle stood over Mr Grant and mercilessly fired his weapon, mortally wounding Mr Grant with a single gunshot wound to the back," the claim writes.
The bullet entered Grant's back and pierced his lung, killing him within hours.
"Oscar yelled, 'You shot me! I got a four-year-old daughter'," friend Fernando Anicete recalled to US network NBC.
Footage of the incident shows officers placed handcuffs on Grant after he had been shot.
Police sources have speculated to US media that Mehserle may have intended to shoot Grant with a Taser, but mistakenly drew his pistol instead.
But Mr Burris said the officer had no justification in even drawing a Taser, let alone a gun, because Grant had co-operated with police from early on and encouraged his friends to do the same.
Police had also not yet confirmed whether Mehserle was carrying a Taser, he added.
Police spokesman Gary Gee implored the public to be patient as authorities carried out their investigation, saying the officer in question had been placed on paid administrative leave.
He did not comment on how authorities would respond to the lawsuit.
Grant's mother Wanda Johnson wept as Mr Burris spoke to the media about her son, a butcher at a small supermarket who friends said loved his daughter dearly and was maturing as he got older.
A funeral service is due to be held at 11am local time on Wednesday (6am AEDT tomorrow).