A Texas Army National Guard soldier who was assigned to Operation Lone Star died in a non-mission-related incident Monday, the Texas Military Department announced.

The soldier, whose identity hasn’t been released pending next-of-kin notification, was injured in Brackettville, some 30 miles east of Del Rio and the Mexico border, and airlifted to a hospital in San Antonio, the department said.

“We are deeply saddened by this loss,” Maj. Gen. Tracy Norris, adjutant general for the Texas Military Department, said in a prepared statement. “We are focused on supporting the soldier’s family and are providing all available resources.”

The department didn’t release details of how the soldier died, but the Army Times reported that the soldier was killed in an accidental shooting.

According to the documents obtained by the Army Times, the soldier was sitting in the front seat of a parked car and handed a gun to another soldier in the back of the car. The gun fired during the handoff, and a round went through the seat and hit the first soldier’s back, the Army Times reported.

Gov. Greg Abbott said Monday that the Texas Rangers are investigating.

“Our hearts are heavy at the news of the tragic death of a Texas Guardsman stationed at the border as part of Operation Lone Star,” Abbott said. “We grieve for the soldier who died and lift their family and loved ones up in prayer.”

Operation Lone Star is a hot-button political issue ahead of the March 1 primary. Republican challenger Allen West and Democratic hopeful Beto O’Rourke have accused Abbott of sending thousands of Texas National Guard troops to the border without proper equipment, regular paychecks or a clear purpose. Both raised alarm over several soldier suicides, which were first reported by the Army Times.

Since then, Abbott has continued to tout the mission, which he says is needed to stop the flow of migrants across the state’s southern border.

Texas Military Department leaders said they’re working to fix the issues, including the pay problems.

The GOP-controlled Legislature has poured about $412 million into the border mission.

Several thousand Texas National Guard soldiers are deployed to the U.S.-Mexico border, though the exact number is not clear.