Mr. Collins’s lawyer, John B. Stella, maintained from the start that his client was a victim who wrestled the gun away from Mr. Cambell and then used it to defend himself.

The investigation took an unusual turn in January, when the federal authorities arrested Mr. Campbell on firearms charges in connection with the shooting. His arrest came after investigators tracked the gun used to shoot Mr. McPhatter to a straw purchaser in Florida, who had sold the gun to an associate of Mr. Campbell’s, according to a federal criminal complaint. The associate passed the gun on to Mr. Campbell.

New York detectives found a significant amount of Mr. Campbell’s DNA on the trigger, handle and magazine of the gun, a 9-millimeter Kel-Tec pistol, while smaller amounts of DNA from Mr. McPhatter and Mr. Collins were found, suggesting that they had also handled the gun, the complaint said. Mr. Campbell was filmed entering the green room and had the gun before the shooting started, according to the complaint.

Image Ronald McPhatter, 33, was killed when gunfire broke out at the Irving Plaza nightclub.

The gun was fired four times in the green room, killing Mr. McPhatter and wounding Mr. Collins, the complaint said. A few minutes later, Mr. Collins was filmed firing the same gun once on the balcony outside the room in the direction Mr. Campbell had fled, the complaint said.

Last month, Mr. Campbell pleaded guilty in federal court to receiving a firearm by interstate commerce with the intent to commit a felony and possessing a firearm as a felon. He faces 20 years in prison when he is sentenced on those charges.

During a bail hearing in federal court, a prosecutor, Hagan C. Scotten of the United States attorney’s office for the Southern District of New York, said the evidence strongly suggested that Mr. Campbell had fired the fatal shot that killed Mr. McPhatter and wounded Mr. Collins as the three men struggled over the gun. Still, the office declined to seek homicide charges against Mr. Campbell.