Authorities said the crash happened at about 1:30 a.m. Monday in Laredo.
Police said a Ford F-150 rolled over, and a 27-year-old passenger was thrown off from the truck. The 21-year-old tried to perform CPR on the victim but was unable to revive him.
Shortly after, witnesses said the driver took his own life. The names of the two guardsmen were not released.
Details regarding the situation are still being looked into.
If you are experiencing suicidal, substance use, or other mental health crises, please call or text the new three-digit code at 988. You will reach a trained crisis counselor for free, 24 hours a day, seven days a week. You can also go to 988lifeline.org.