AUSTIN, Texas (CBSDFW.COM/AP) — Three Texas Parks and Wildlife employees died after their helicopter crashed in a Texas wildlife preserve, the department announced Sunday.

The group was surveying desert bighorn sheep in the Black Gap Wildlife Management Area in West Texas on Saturday when the helicopter went down, according to a statement from the agency.

It was not immediately clear what caused the crash, which Gov. Greg Abbott called a “tragic accident.”

It killed Dewey Stockbridge, a wildlife biologist, Brandon White, a department technician, and Dr. Bob Dittmar, a veterinarian. The helicopter’s pilot, a private contractor, survived and was taken to El Paso for treatment, according to the department.

“Our hearts and prayers go out to the families of the three Texas Parks & Wildlife workers who died in a West Texas helicopter crash while working. Their service to our state is cherished & will never be forgotten,” Abbott said in a tweet.

Lt. Gov. Dan Patrick also released a statement on the crash, saying, “Jan and I are deeply saddened to hear the news of this horrible accident, and our prayers are with the families, friends and loved ones of the victims. We ask all Texans to join us in mourning their passing.”

The Black Gap Wildlife Management Area is near the U.S.-Mexico border in Brewster County, some 300 miles southeast of El Paso.

The Texas Department of Public Safety, the Federal Aviation Administration and the Texas Game Wardens are investigating the crash.

The National Transportation Safety Board said it is investigating the crash of a Bell 206B helicopter but staff are not traveling to the site.

(© Copyright 2020 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)