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A man with a shotgun opened fire on motorists and police who stopped to help with what appeared to be a wreck on a downtown freeway early Sunday, killing two people and wounding a police officer and a Fort Worth man before taking his own life, authorities said.

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 had apparently 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 35E when someone inside the car shot at them. One witness drove off after Salinas 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,"Hale 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 rendered aid, his partner, Senior Cpl. Jerry Poston, was shot. No Dallas police officers are believed to have fired any shots, Hale said."We do not know the exact sequence of [the] shootings and probably never will."

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 other 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. He was at Parkland Memorial Hospital, in stable condition with serious injuries, police said.

A woman at Terrazas'home said the family is still grieving and declined to comment. A phone call to Salinas'home Sunday evening went unanswered. Poston's partner was not identified.

Associated Press reporters Rich Matthews and Thomas Peipert andStar-Telegramstaff writer Cynthia Neff contributed to this report.