The man police say was fatally shot by an off-duty Pentagon Force Protection Agency officer who saw him beating another man in Clinton Friday afternoon was identified as a 20-year-old from Accokeek. Authorities also identified the officer, Alex Thomas.

Prince George’s County police said that Thomas pulled into a Mobil gas station in the 9800 block of Piscataway Road about 1:30 p.m. and saw Ronald Delonte Royal beating a man who was on the ground.

As Thomas approached them, he saw Royal pull out a handgun and point it at the man on the ground, police said. Thomas told Royal that he was a police officer and ordered him to drop his weapon, according to police.

Royal refused and turned toward Thomas, police said. Thomas then fired his personal gun, hitting Royal in the upper body. Royal was taken to a hospital, where he died.

The man on the ground, a 57-year-old, was treated for injuries that were not life-threatening, according to Cpl. Erica Johnson, a police spokeswoman.

Thomas was not in uniform at the time and was carrying a weapon that he owned, according to Terry Sutherland, a Pentagon Force Protection Agency spokesman.

Sutherland said Friday that the agency’s internal affairs department will investigate the shooting, as is routine in such cases.

Thomas, who will have served eight years in September, might be given a few days of personal leave, but it would not be punitive, Sutherland said.

Royal was scheduled to appear in Prince George’s County Circuit Court in September in connection with robbery, assault and gun charges stemming from a July 7 incident, according to state court records. A man listed as Royal’s attorney in a different case did not return a call for comment.