Police say Walker was on meth the morning of September 1 when Sgt. Johnson approached his car at 132nd South and Fort St. in Draper.
Walker was parked there because his Volvo was out of gas and had a flat tire from driving over a curb, stop sign and large rock just a few blocks away.
Police say Walker shot Johnson through the open passenger side window and hit him in the chest.
Johnson took off and Walker fired more shots that hit the back of the marked police vehicle.
Johnson put out a call of shots fired before losing control and crashing into a tree about two blocks from where he was shot.
Investigators say Walker then shot his girlfriend, Traci Vaillancourt before turning the gun on himself.
Salt Lake County District Attorney Sim Gill says Johnson was just stopping to help and never saw it coming. Gill says because of the aggravate nature of the case, Walker could pay with his life if convicted of the charges.