The company reopened three of the buildings after emergency officials deemed them safe.
Two individuals may have been exposed to sarin but are not now showing signs or symptoms of exposure, Reuters reported, citing a local fire department official.
Four of the social media company's buildings were evacuated on Monday and two people were checked for possible exposure to the compound that attacks the nervous system and can be fatal.More news: Kaley Cuoco Returns With Post-'Big Bang Theory' Project
"Right now we don't have anybody that has any symptoms", he said. The possibility exists that the detection was a false positive, and an initial open air test suggest that to be the case.
The FBI has responded to Facebook's request for the hazmat team.
"Authorities have not yet identified the substance found". A preliminary report suggested that at least two people might have been exposed, but authorities updated to say that no employees were affected. "We're just doing verification".
The nerve agent sarin has been detected at a Facebook mailroom in California.More news: Reifer reveals West Indies' World Cup soul-searching
"The facility tests all of the packages that come in and they had a positive test, so they just initiated their standard protocol", he said.
Sarin is a "human-made chemical warfare agent classified as a nerve agent", according to the Centres for Disease Control and Prevention.
Exposure to large doses of sarin can lead to symptoms including loss of consciousness, convulsions, paralysis and respiratory failure possibly leading to death, according to the CDC's website.More news: Marcus Rashford signs new four-year deal at Manchester United
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