A bomb squad converged on downtown San Mateo this morning after police stopped a driver wearing a gas mask and found a hand grenade in his car, police said.
Police spotted the costumed man driving on Fourth Avenue at about 9:30 a.m.
Officers stopped the driver -- later identified as Gilberto Masias, of San Mateo -- in the first block of East Fourth Avenue, and noticed a grenade in his car, police said.
Out of "an abundance of caution for public safety," Masias was detained, and a half-block area around the grenade was evacuated while a San Mateo County sheriff's bomb squad responded, police said.
The grenade was determined to be real, but inactive, police said.
Masias, an employee at a local costume shop, was cited for possession of an inactive grenade and released.
San Mateo police spokesman Sgt. David Norris issued a statement this afternoon reminding community members to be mindful when choosing and wearing costumes in advance of Halloween.
"Costumes with very realistic weapons can create fear and safety concerns among the community members, and may provoke a response by law enforcement," Norris said.