At Least 5 Dead and Up to 20 Injured in Shootings Near Odessa, Tex.

HOUSTON — At least five people were killed and 21 others injured in shootings near the West Texas cities of Odessa and Midland on Saturday, as one or two gunmen in a hijacked postal van and a pickup truck opened fire on residents, motorists and shoppers, the authorities said.

The two attacks in the two cities terrified a region with a combined population of 263,000 at the start of Labor Day weekend. Police officers and state troopers tried to keep drivers off the highways.

Chief Michael Gerke of the Odessa Police Department said at a news conference on Saturday evening that the gunman, a white male in his mid-30s, was dead, and that three law enforcement were shot. He did not give a motive, but said that the attack began as a traffic stop.

The chief added that there were reports of a second gunman but that they were not confirmed, attributing the sightings to the chaotic situation.

“We do believe we have the threat contained but I can’t be 1,000 percent sure of that,” he said.

The mayor of Midland, Jerry Morales, said the shootings began shortly after 4 p.m. local time, with motorists targeted on Interstate 20 and Highway 191, which connects Midland to Odessa, Tex.

Mr. Morales added that a rifle was used in the shootings, though he could not be more specific about the type or number of firearms involved.

As the situation was unfolding, the police in Odessa said on Facebook that a gunman — or possibly two — was driving around “shooting at random people.”

Ernest Villanueva, 24, said he was driving westbound on Interstate 20 from Midland to Odessa when he approached two vehicles riddled with bullet holes. One was a blue pickup truck and the other was a silver sedan.

“I was ready to help in case there were injured people,” said Mr. Villanueva, a Marine Corps veteran.

The vehicles were empty, according to Mr. Villanueva, who said he saw Midland police vehicles heading toward Odessa.

“I think it’s so surprised it happened so soon with the recent events of El Paso,” Mr. Villanueva said of the mood in Texas.

The University of Texas of the Permian Basin said on Twitter that law enforcement authorities were trying to “locate and contain a shooter.”

The university directed that students “stay in your dorm or office and do not open your doors.”

The federal Bureau of Alcohol, Tobacco, Firearms and Explosives said on Twitter its agents were responding to the scene.

Mariel Padilla contributed reporting.