The claim, filed by attorney John Burris on behalf of Wanda Johnson, Oscar Grant III's mother, and Sophina Mesa, the mother of Grant's 4-year-old daughter, also names the BART police officer who shot Grant as Johannes Mehserle.

Grant, who worked as a butcher at an Oakland grocery store, "was unarmed and offered no physical resistance to BART police officers" before being shot shortly after 2 a.m. on Jan. 1, according to the claim.

Grant was riding BART back to the East Bay "after a night of celebrating New Year's Eve in San Francisco" when an altercation erupted on his train, prompting BART police to meet the train at the Fruitvale station to quash the fight, the claim states.

Mehserle and other officers, whose names aren't mentioned in the claim, ordered Grant and other passengers to get off the train, according to the claim.

The claim states that Grant complied and stepped off the train, but that an unnamed "Latino officer" grabbed Grant's upper body, threw him against a nearby wall and kneed him in the face.

Grant then dropped to his knees and put his hands up "in an effort to demonstrate that he was submitting to the Latino officer's thuggish display of authority," according to the claim.

But the officer dug his knee into Grant's back, causing Grant to "yell out in agony," the claim states.

Grant feared for his life and "made a valiant effort to de-escalate the situation by appealing to the officer's sense of humanity by telling the officer that he had a 4-year-old daughter" and asking the officer not to use a Taser gun on him, according to the claim.

The claim alleges that Mehserle, who was standing nearby, kneeled down and restrained Grant's hands, then "inexplicably" stood up, drew his firearm and pointed it directly at Grant's back.

The claim states, "Without so much as flinching, Officer Mehserle stood over Mr. Grant and mercilessly fired his weapon, mortally wounding Mr. Grant with a single gunshot sound to the back."

Video footage of the shooting and witness statements indicate that after Grant was shot officers placed him in handcuffs "without any justification," the claim states.

BART spokesman Linton Johnson has not returned calls for comment today.

At a briefing at BART headquarters on Monday, Johnson told reporters that BART officials won't have much to say about the shooting until their investigation is completed, a process he said will take at least several weeks.

"We urge the public to be patient with us," Johnson said Monday. "It's frustrating to us because people are getting upset about what happened, but we owe it to the public, the family (of Grant) and the police officer to get all the facts."

"We want to investigate this quickly but we don't want to have a rush to judgment," he said.

Burris said that if BART rejects the claim, he will file a lawsuit after 45 days.

Burris said he believes Grant was cooperative during the incident and that BART police officers "escalated the situation."

Some police use-of-force experts who have looked at footage of the incident have said that Mehserle may have meant to fire a Taser at Grant but instead mistakenly shot him with a gun.

But Burris said, "I'm not sure if that's true" and pointed out that BART hasn't yet said one way or the other if Mehserle had a Taser during the incident.

Burris said he doesn't think Mehserle had any justification to use even a Taser on Grant.

He said that if Mehserle deliberately shot Grant with a gun, the officer should be prosecuted for second-degree murder.

Burris said if Mehserle had intended to use a Taser and mistakenly fired his gun, the officer should be prosecuted for involuntary manslaughter.

"It's negligence if you pick the wrong weapon," Burris said.

Mehserle is on paid administrative leave while the incident is investigated by BART and the Alameda County District Attorney's office.

District Attorney Tom Orloff, who has the power to file charges against Mehserle if the thinks they are warranted, said the investigation by his office may take several weeks because it will take time to complete the autopsy protocol and toxicology reports.

In addition, his office may need to re-interview witnesses, Orloff said.

A memorial service will be held for Grant at 11 a.m. Wednesday at Palma Ceia Baptist Church, located at 28605 Ruus Road in Hayward.