A machine at a Facebook mailing facility in Menlo Park, California, alerted employees Monday that a package might contain sarin, according to fire officials.
No employees have been exposed to the substance, the Menlo Park Fire District said.
The Centers for Disease Control and Prevention defines sarin as a "human-made chemical warfare agent classified as a nerve agent."
Facebook runs all of its mail and packages through a machine that can detect dangerous substances, according to the fire district. That machine notified workers that the package in question might contain the substance.
Fire officials initially reported that two people may have possibly been exposed.
A Facebook spokesperson said four buildings in the area were initially evacuated. Only one building remains evacuated. The others have been deemed safe.
An investigation into the incident is ongoing.