Neighbourhood watch volunteer George Zimmerman followed and confronted the black teenager he is accused of murdering after a police dispatcher told him not to, prosecutors said.
Zimmerman, 28, made his first court appearance in Florida on Thursday, accused of second-degree murder in the shooting of 17-year-old Trayvon Martin.
The prosecution's case, outlined in an affidavit filed in support of the charges, appeared to contradict Zimmerman's claim that Trayvon attacked him after he had turned away and was returning to his vehicle.
In the affidavit, prosecutors also said that Trayvon's mother identified cries for help heard in the background of an emergency services call as her son's.
The account of the shooting was released as Zimmerman appeared at a four-minute hearing in a jailhouse courtroom, setting in motion what could be a long, drawn-out process or an abrupt and disappointingly short one for the Martin family because of the strong legal protections contained in Florida's "stand your ground" law on self-defence.
During the hearing, Zimmerman wearing a grey prison jumpsuit, spoke only to answer "Yes, sir" twice after he was asked basic questions from the judge, who was not in court but on closed-circuit TV. The defendant's hair was shaved down to stubble and he had a thin goatee. His hands were shackled.
He did not enter a plea - that will happen at his arraignment, which was set for May 29. Zimmerman's lawyer, Mark O'Mara, has said his client will plead not guilty. A bond hearing for Zimmerman will probably be held on April 20, Mr O'Mara said later.
To prove second-degree murder, prosecutors must show that Zimmerman committed an "imminently dangerous" act that showed a "depraved" lack of regard for human life. The charge carries a mandatory sentence of 25 years in prison and a maximum of life.
The special prosecutor in the case, Angela Corey, has refused to explain exactly how she arrived at the charge. But in the affidavit, prosecutors said Zimmerman spotted Trayvon while patrolling his gated community in Sanford, got out of his vehicle and followed the boy.
Zimmerman told authorities that Martin attacked him as he was going back to his vehicle, punched him in the face, knocked him down and began slamming head against the pavement.