Las Vegas ? A gunman who opened fire with a shotgun at a federal building Monday, killing one security guard and wounding a U.S. marshal before he was shot to death, was upset over losing a lawsuit over his Social Security benefits, law enforcement officials told The Associated Press.

The two officials, who spoke on condition of anonymity because they were not authorized to discuss the case, said 66-year-old Johnny Lee Wicks opened fire with a shotgun at a security checkpoint, touching off a gunbattle with deputy U.S. marshals.

Although the investigation is continuing, the officials said preliminary evidence pointed to Wicks’ anger over his benefits case as the motive for the shooting.

Authorities also were investigating the cause of a fire that damaged Wicks’ modest one-bedroom apartment in a 90-unit seniors complex three miles northwest of the scene of the shooting.

The gunman opened fire at the courthouse at about 8 a.m. Gunfire lasted several minutes, with shots echoing around tall buildings in the area more than a mile north of the Las Vegas Strip.