DALLAS (AP) — Investigators are offering a $50,000 reward as they search for a suspect in the fatal shooting of a U.S. Postal Service worker who was found dead in his mail truck in Dallas.

Federal investigators announced Tuesday they were offering the reward for information leading to an arrest and conviction following the death of 58-year-old Tony Mosby.

Dallas police say Mosby was shot early Monday while driving a mail delivery truck on Interstate 30 near downtown Dallas. The shooting took place in an area near two major postal service complexes.

Officials are also trying to determine a possible motive for the attack.

The reward is being offered by the U.S. Postal Inspection Service, the law enforcement and security arm of USPS. The agency is working with Dallas police on the case.