Originally developed in 1938 in Germany as a pesticide, sarin is defined as a "human-made chemical warfare agent classified as a nerve agent".
"Authorities have confirmed test results were negative for any potentially risky substance and the buildings have been cleared for repopulation".
An item sent through one of the facility's mail scanning machines tested positive for sarin, Menlo Park Fire Marshal Jon Johnston said.
It's a highly toxic, clear and odourless liquid that can disrupt the functioning of the nervous system.
Facebook spokesman Anthony Harrison said the company evacuated four buildings following the positive test and was co-operating with police.
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"As part of Facebook's routine mail security screening process, we identified a potentially risky substance", said Anthony Harrison, director of corporate media relations at the company. "The safety of our employees is our top priority and we will share additional information when it is available".
It was not immediately clear how many people were forced to leave the affected buildings.
There's a report that a test at one of its mailing facilities was positive for the nerve agent sarin.
CBS News confirmed that the Federal Bureau of Investigation is responding to Facebook's headquarters at the request of the hazmat team.
According to the Los Angles Times, at around 11 am Monday a package scanner in a Facebook mailroom detected traces of sarin prompting an evacuation of four buildings.
Exposure is possible through skin or eye contact, as well as inhalation.