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Investigators are still trying to piece together why Draper police Sgt. Derek Johnson was shot and killed Sunday morning.

Johnson, 32, pulled up to a silver Volvo parked at an odd angle at 13200 S. Fort St., where police suspect a 35-year-old man pulled a handgun and shot the decorated officer. Investigators also say that a 34-year-old woman who was with the man was also shot before he turned the gun on himself. While Johnson died at Intermountain Medical Center, the others, who have not yet been publicly identified, survived and remain in critical condition.

Salt Lake County Sheriff Jim Winder joined a news conference at Draper City Hall on Monday afternoon, where Johnson's family stood nearby holding each other, to field questions about the Unified Police Department's ongoing investigation into the shooting. With one witness dead and the other two still unable to speak to police, some answers, including the motive, remain elusive. But investigators are not giving up.

"We absolutely want to ensure the family standing behind us that [those responsible] will be brought to justice," Winder said.

UPD is working on obtaining a warrant for the Volvo to search it for evidence. Draper Police Chief Bryan Roberts revealed Monday that the man and woman are transients, with local roots, who had been living in the Volvo and moving around the Salt Lake Valley. Roberts could not say for how long they have been transients.

Winder added that they have "routine" criminal histories, though he did not elaborate.

The Volvo had sustained some moderate damage after the couple struck rocks and fences near 900 East and 13800 South before the shooting. They had kept going until they came to a stop at 13200 S. Fort St., where Johnson found them. Investigators have not determined whether the woman was shot before or after Johnson and have not released how many shots were fired in all.

Winder reiterated that Johnson never left his vehicle or fired back during the ambush.

The police encourage anyone who may have information about the shooting or may have seen the Volvo on Saturday night or Sunday morning to call UPD at 801-743-7000.

"We are still interviewing witnesses, processing evidence and are trying to establish a motive for this horrible crime," Roberts said. "… We are doing everything we can to maintain the integrity of this case and have a solid case to present to Salt Lake County District Attorney's Office."

Though the shooting was only a day ago, to Johnson's father, Randy Johnson, it "seems like a long time." He spoke at Monday's news conference about how much they loved and will miss Johnson, who leaves behind a wife and young son.

"We are so thankful we had him the years that we did," Randy Johnson said.

Though Johnson's family did not feel ready to attend a candlelight vigil at city hall Sunday night, Randy Johnson said they have seen the U.S. flags and yellow, black and blue ribbons tied around trees throughout the city. He thanked everyone for their support.

Randy Johnson also related how his son approached the job he loved: "If I treat people with respect ... I'll receive that same respect."

Derek Johnson had joined the police academy soon after high school graduation and was made a full-time Draper police officer in 2004. He was then promoted to sergeant in December 2012, working in patrol, community policing and investigations. He also received a community policing award that year.

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