Brian Schober was driving to his job in Yuma on Thursday when he came upon the bright lights of a stopped police SUV and a man waving his hands, asking for help.
When Schober got out of his pickup truck, he saw two men on the pavement, both shot.
One was Ed Andersson, an injured Arizona state trooper. The second was a man authorities said had been beating the trooper after shooting him twice. The man who flagged Schober down had fatally wounded that attacker minutes before Schober stopped to see what was going on.
A woman was on the ground, too, having been ejected from a car that had crashed off the side of the highway. She died from her injuries.
Schober, a 44-year-old engineer, got on the trooper's portable radio and calmly called for help.
"Hello, officer down, officer down outside Tonopah," Schober says. "Come in, please. This is a civilian. He's shot on I-10 ... He's in real bad shape. Please send air support, helicopter, please."
Schober tells the dispatcher there are two other people who need medical attention.
Schober sent the good Samaritan's fiancee, who had placed a 911 call before Schober arrived, after the first aid kit in the trooper's SUV.
"I think some of my EMT training came back. It's been 25 years, but some of it probably came back instinctively," Schober told CNN on Friday.
Schober cleaned the officer's head wounds from the beating and searched for where a bullet hit the trooper's body.
The first motorist, meanwhile, kept an eye on the unidentified suspect, who died at the scene just a few feet from the trooper.
"The timing was everything. If the civilian had been there any later the cop would probably have bled out," Schober said. "If I hadn't been driving that way at that time, the authorities probably wouldn't have made it on time and the cop would have died. It was a two-pronged scenario, and we were there at that time for a reason."
Other emergency responders arrived at the scene about 5 a.m., Schober said.
"Once medics showed up with flashlights, we found the wound and saw the extent of his head injuries," he said. Authorities said the trooper was actually hit twice -- in the shoulder and chest.
The trooper's surgery went well, Col. Frank Milstead, the director of the Arizona Department of Public Safety, tweeted. "He & family ask for privacy during recovery. Appreciates outpouring of support," Milstead wrote.
Authorities do not know why the suspect shot the trooper, who was responding to a report of shots fired and happened upon the rollover crash. He was setting up flares when the suspect shot him, officials said.
Moments later, the first motorist stopped at the scene. He had a gun in his truck and shot the man as he pistol-whipped the trooper, authorities said. The two witnesses were questioned separately until 3 p.m., Schober said.
None of the other people involved in incident have been identified. The motorist who shot the suspect asked authorities to tell the media he doesn't want to give interviews.