Witnesses described Pirone as the "crazy cop" who was "very agitated," shouting orders speckled with profanity as he rushed onto the platform in response to reports of a fight on the train.
The investigation found that Grant didn't fight back when Pirone rushed over to him, shoved him against a wall and punched him in the head.
After Grant sat down, Pirone kneed him in the face, a strike the internal investigation found to be "punitive" and unjustified.
Pirone used a racial epithet during a back-and-forth with Grant, reportedly repeating the words "bitch-ass n----."
The former officer continued to struggle with Grant and was kneeling on the back of Grant's head and neck until shortly before Mehserle stood up, drew his gun and fired a single round into Grant's back.
The investigation also found that Pirone lied repeatedly about the events that led to Grant's death.
The investigation concluded:
"Pirone’s repeated, unreasonable and unnecessary use of force; his willful and reckless conduct that endangered the safety of the public and his fellow officers; his failure to be forthcoming about the true events; his changing and shifting stories; his manifest lack of veracity; his professionally inappropriate demeanor; his use of a racially offensive word; and his excessive use of expletives, warrant a recommendation that Officer Pirone be terminated from his employment with BART."
BART subsequently fired Pirone, and an arbitrator upheld his termination in 2014.
Grant's family and their supporters have recently renewed calls for another prosecution in the historic case, focusing largely on Pirone.
"That report states that Tony Pirone created the chaotic atmosphere on that platform," Cephus 'Uncle Bobby X' Johnson, Grant's uncle, said in an interview Monday. "It cites everything that he did that created the volatile situation that caused the murder of Oscar Grant."