A San Bruno resident dropped off the box of items, which included a BB gun and fireworks, at the Police Department at 1177 Huntington Ave. at about noon, San Mateo County Office of Emergency Services district coordinator Brian Molver said.
He said the vial is in a plastic Ziploc bag but that responders are being cautious because "we're not taking any risks."
Authorities contacted the resident after a police officer found the vial, and the resident told police the box had been in a garage for several years, Molver said.
The vial was in an evidence room at the San Bruno Police Department as of 1:30 p.m., and police were waiting for the county's hazardous materials team to respond and determine what the substance is.
Molver said no one was evacuated. The San Bruno Police Department is located near the San Bruno BART station and the Tanforan Mall.