LAWRENCEVILLE, Ga. -- Authorities said a man found dead in Duluth died of a gunshot wound to the head.

Gwinnett County police released the cause of death of Stone Mountain businessman Marc Stewart in a statement Tuesday. Investigators are treating the death as a homicide.

SLIDESHOW: Marc Stewart

Police say Stewart's body was found Saturday night inside a Ford Explorer. The 38-year-old businessman hadn't been seen since the morning of Feb. 11 when he left for work.

Stewart was president and chief executive officer of Island Coast Entertainment, an Atlanta-based event planning company.

Stewart’s bank card and cell phone had been used since he disappeared, family friend Kiyomi Rollins told Channel 2.

Stewart’s wife, Andrea White, received an e-mail alert from Bank of America stating "irregular check card activity" had been spotted on Stewart’s account, said Rollins.

The alert came three hours after White received a call from her husband saying he was on his way home, Rollins said.

The motive and circumstances surrounding his death remain under investigation.