A Mississippi teenager was arrested Monday night for allegedly gunning down an on-duty police officer who was set to retire at the end of the year, authorities said.

Darian Tawan Atkinson, 19, is expected to be charged with capital murder for the fatal shooting of Biloxi Police Officer Robert McKeithen, law enforcement officials said at a press conference.

The 24-year veteran officer was ambushed in the parking lot of the Biloxi police headquarters Sunday at around 10 p.m. and shot multiple times, Biloxi Police Chief John Miller told reporters.

It wasn’t immediately whether McKeithen was specifically targeted by the gunman or randomly selected.

Prior to the shooting, the killer was spotted on surveillance video casually strolling into police headquarters and grinning, according to footage released by the Gulfort Police Department.