HOUSTON -- The night manager of a northwest Houston fast-food restaurant was shot and killed when he took a break outside late Thursday, KPRC Local 2 reported.

Houston police said John Marx, 27, was killed at a Whataburger in the 2400 block of Gessner Driver near Emnora Lane at about 11:30 p.m.

"An employee stepped out and a few minutes later the other employees heard gunshots," Officer E.R. Carstens said.

The manager had the keys to the restaurant and the other employees were locked inside.

Employees called out for Marx after hearing the gunshots but he did not respond, officials said.

Officials said a customer at the drive-through window was asked to check behind the building. The witness observed Marx on the ground blocking the back door.

None of the other employees was injured or threatened.

Police said the killer may have known the manager would have been behind the restaurant smoking a cigarette at that time.

The motive for the shooting is not known, officials said.

Investigators have not released information about possible suspects.

Anyone with information is asked to contact Houston Police Homicide Division at 713-308-3600 or Crime Stoppers at 713-222-TIPS.