District Attorney Larry Krasner said during a news conference on Thursday that the gun believed to be used in the incident was recovered and is being tested.
Krasner says the man believed to have fired at the off-duty officer has spoken with authorities and shared his version of the deadly confrontation.
His name has not been released. So far, no charges have been filed.
The officer, 29-year-old Alhassane Barry, was a two-year veteran of the force.
He was shot and killed outside an after-hours lounge on the 7200 block of Elmwood Avenue in Southwest Philadelphia in the early morning hours of Sunday, Aug. 16.
Investigators say video from inside the lounge shows the off-duty officer and a man arguing inside before the confrontation escalated outside.