SAN MATEO -- A Halloween costume store worker in a gas mask and carrying a dummy hand grenade prompted a bomb scare Monday morning that ended with a citation for the clerk, police said.

An officer on patrol spotted a gas mask-clad driver around 9 a.m., pulled the car over on the first block of East Fourth Avenue and noticed a hand grenade, San Mateo police Sgt. Dave Norris said.

Police decided to detain the man and "isolate" the grenade while they waited for the bomb squad. A half-block area around the grenade was evacuated. The bomb squad later determined the grenade was real, but inactive.

Police cited the driver, Gilberto Masias, 29, of San Mateo, who works at a costume shop, for having the inert grenade and then released him.

"Costumes with very realistic looking weapons can create fear and safety concern among community members, and may provoke a response by law enforcement," wrote Norris. "Please be smart with your costume choice."

Contact Joshua Melvin at 650-348-4335. Follow him at Twitter.com/melvinreport.