Authorities say they may never know why 20-year-old Nick A. Salinas of Cedar Hill started shooting. He killed himself with the shotgun used in the slayings.
Dallas police identified the victims as Robert Daniel Langston, 22, of Duncanville, and Jesus Reyes Terrazas, Jr., 24, of Dallas. Both apparently had stopped to offer help.
"It appeared that each of these victims thought that they were providing roadside assistance to an accident victim when they were brutally shot without any motive that we know of," said Lt. Vernon Hale, a Dallas police spokesman.
Witnesses told investigators they stopped to help a motorist on the side of Interstate 35 when someone inside the car shot at them. One witness left in his car after the shooter pointed a gun at him, police said.
Terrazas was part of a group of four who saw what they believed was an accident before officers arrived and turned around to help. Someone in the group returned fire, Hale said.
"Certainly I'm sure he was just trying to protect himself," he said.
Officers spotted two people lying on the side of the road and stopped to help, unaware of the shootout, police said. As one officer aided a person on the ground, his partner, Senior Cpl. Jerry Poston, was shot.
"No officers from the Dallas Police Department fired any shots that we know of," Hale said.
Poston, a 17-year veteran of the department, was hospitalized at Methodist Central in serious but stable condition, police said. His family asked that no information about him be released, said Lynette Wilkinson, a spokeswoman for Methodist Dallas Medical Center.
Police identified the other wounded man as Barry David Smith, 30, of Fort Worth. Police said he was in stable condition with serious injuries.
A woman at Terrazas' home said the family was still grieving and declined to comment. A phone call made to Salinas' home Sunday evening went unanswered. Poston's partner was not identified.