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.
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"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.
