District Attorney Sim Gill said the death penalty is "on the table" in the case, but his office would wait until after the preliminary hearing and within 60 days of Walker's arraignment to make a final decision.
"[Johnson] wanted to do nothing but help out," Gill said. "As he stops to assist a car in distress, this tragedy unfolds."
Walker was booked into the Salt Lake County jail Wednesday afternoon after being released from the hospital, where he was being treated for a self-inflicted gunshot wound.
Walker, who has no permanent address, also was charged with one count of attempted homicide for allegedly trying to kill his girlfriend Traci Vaillancourt, 34, who also has no permanent address, during the same Sept. 1 shooting.
In addition, Walker was charged with three counts of discharge of a firearm and one count of possession of a weapon by a restricted person, all third-degree felonies.
Vaillancourt also was charged Wednesday in 3rd District Court with two counts second-degree felony counts of obstructing justice for allegedly lying to investigators about the incident.
According to charging documents, Johnson, 32, was in a marked Draper City police vehicle at about 6 a.m. when he stopped at 13200 South Fort Street and noticed Walker standing outside his stopped Volvo.
Authorities said Walker had stopped because he ran out of gas and the passenger-side front tire had gone flat because he had driven over a curb, a stop sign, a large rock and then through at fence at 13800 S. 933 East, court records show. He then continued driving a few blocks away to the area of 13200 South, police said.
When Johnson pulled up near the Volvo to see if he could help, Walker allegedly shot Johnson through the open front passenger side window of Johnson's police vehicle.
Johnson was struck in the chest by the bullet and tried to drive away northbound. Walker, who was allegedly under the influence of methamphetamine at the time, fired at least three more shots, which struck Johnson's marked vehicle, court documents state.
Johnson used his police radio to call "shots fired," but then lost control of his vehicle and crashed into a wooded area about two blocks north of where he was shot.
The medical examiner later determined Johnson died from a gunshot wound to the chest.
Gill said Johnson was wearing a bulletproof vest, but the bullet struck in an area the vest did not cover.
During the episode, Walker turned the gun on Vaillancourt, striking her in the upper-right back, court records show.
After shooting Johnson, Walker then shot himself.
Walker was hospitalized for his injuries. Police, who have previously said they expected Walker to survive his injuries, declined on Wednesday to comment on his current condition.
Court records indicate a $2 million cash-only warrant was issued Wednesday for his arrest.
Vaillancourt was released from a hospital late Saturday and booked into the Salt Lake County jail.
After Vaillancourt was treated for her injury, police said they tried to question her on Sept. 1, 4, and 7. But on Sept. 4, she initially denied seeing Johnson and said she did not see who was shooting or know who shot her.
During a Sept. 7 interview, police said Vaillancourt told police how she was positioned at the time of the shooting, but the location of her wound, the directionality of the wound and the lack of physical evidence, did not support her statement, court records alleged.
Police said on Sept. 7, Vaillancourt admitted that she did see a marked police vehicle pull up next to the Volvo and heard several shots fired in quick succession, one of which hit her.