A recently unsealed file noted how transit officer Anthony Pirone ‘started a cascade of events that led to the shooting’ of the unarmed black man

A police officer involved in the 2009 killing of Oscar Grant on an Oakland train platform repeatedly lied to investigators and had punched the unarmed 22-year-old without justification, according to newly released records.

The report on the New Year’s Eve killing, which sparked national police accountability protests, was disclosed this week following journalists’ requests under a new California police transparency law. The previously sealed internal file, written 10 years ago, documented how the Bay Area Rapid Transit (Bart) officer Anthony Pirone “started a cascade of events that ultimately led to the shooting”. Pirone called Grant the N-word while detaining him, hit him in the face in an “unprovoked” attack, and later gave a series of false statements contradicted by videos, investigators said.

The death of the young father was one of the first major US police brutality cases in which cellphone footage went viral, prompting widespread outrage years before the Black Lives Matter movement. The killing was later made famous by Ryan Coogler’s 2014 film Fruitvale Station, named after the site of the death.

The officer who shot Grant in the back, Johannes Mehserle, was found guilty of involuntary manslaughter in a rare criminal trial over a killing by law enforcement.

The newly disclosed report, written by a law firm hired by Bart to investigate the case amid intense scrutiny in 2009, offers new insights into the authorities’ formal conclusions in the months after Grant’s death. It also reveals Bart’s understanding of the ways Pirone escalated the situation and then lied about Grant’s actions in an effort to paint him as an aggressor.

Pirone, responding to reports about a fight on the train, ordered Grant off the train in an “aggressive” manner while shouting profanities, and then forced him to sit on the ground, the report said. The Bart officer later claimed that Grant attacked him, saying: “He tried to punch me … He started kneeing me, then he kicked me and that’s when I put up a forearm to … the upper region of his body.”

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But the report noted that “video reveals a different story”, saying: “Pirone approached Grant, grabbed hold of him and pushed him against the wall … he [then] appears to have struck him one time in the head or facial area.” There was no evidence that Grant “kneed Pirone in the groin”, as the officer claimed.

Pirone later said he felt as if he was “fighting for my life” with Grant, but investigators wrote that “none of this appears to have happened during the video”. Pirone also “kneed Grant in the face”, which the officer did not report, and an autopsy revealed Grant suffered a hemorrhage, investigators said.

“Pirone used force against Grant as a first resort and even then the use of force … did not appear reasonable, justifiable or excusable,” the report continued, saying his “willful and reckless conduct … endangered the safety of the public”. The officer was “responsible for setting the events in motion that created a chaotic and tense situation on the platform”, and the report said he should be terminated in part due to his repeated “untruthfulness”.

Pirone was fired after the investigation was finalized, a Bart spokesman noted on Thursday. Pirone’s attorneys could not be reached for comment.

The report also revealed that the investigators did not believe Mehserle’s argument that he meant to draw his Taser and not his gun. The report said “enhanced video” suggested Mehserle was “intending to pull his firearm and not his Taser”.

Adante Pointer, one of the civil rights lawyers who represented Grant’s family, told the Guardian on Thursday that the report was further evidence of Bart’s mishandling of the case.

“Bart spent a bunch of public money to defend Mehserle’s actions, to defend the actions of the cops,” he said. “They should have just been forthright with the public and released the details … They should’ve done the right thing and admitted that what the officers did on that platform was wrong.”

Pointer continued: “They made the family fight for years to prove that what happened to their loved one was unjust … You dragged this family through so much.”

A Bart spokesman, James K Allison, said in an email that the shooting was a “tragic loss of life”, adding: “Since the tragedy, Bart and its police department have worked diligently to ensure that our officers receive the proper training, support and oversight necessary to respond appropriately and effectively to challenging situations.”

John Burris, another lawyer for Grant’s family, said the report was a further reminder that police might have gotten away with fabrications if the cellphone footage did not exist and make clear Pirone was the “perpetrator”.

“For the people throughout the nation who believed Oscar Grant’s life was wrongly taken, this is vindication,” he added.