A man wearing a gas mask while driving Monday morning created an alarming scene in downtown San Mateo after officers reportedly found an inactive grenade inside his vehicle.

Just after 9 a.m. on East Fourth Avenue, police said, 29-year-old Gilberto Masias was stopped by officers who said they were alarmed by his apocalyptic-looking headgear. The gas mask might have been part of a Halloween costume, as police said Masias works at a local costume store.

But wearing the mask in broad daylight — and also a full month before Halloween — forced officers to conduct a “high-risk” traffic stop, according to police.

Masias was found in possession of “a realistic-looking grenade,” police Sgt. Dave Norris said. The grenade was in fact real, Norris said, but was found to be inert.

Still, the incident caused quite a stir. A bomb squad investigation forced the evacuation of a half-block area surrounding the grenade, Norris said. Masias was cited for possessing an inert grenade, he added.

Police warned residents “to be mindful” when choosing Halloween costumes.

“At even a short distance, expert police officers may not be able to tell if a realistic-looking weapon is real or not,” Norris said. “Please be smart with your costume choices.”

maldax@sfexaminer.com