Fire officials initially reported that at least two Facebook employees may have been possibly exposed to the packet in question. "Everything is contained and quarantined within the building now", said Menlo Park Fire Marshal Jon Johnston.
Facebook's Silicon Valley headquarters was given the all-clear on Tuesday after authorities determined that an alert for the deadly sarin nerve agent had been a false alarm.
The major hazmat response and drama Monday in Menlo Park was allegedly much ado about nothing, but what would cause a screening machine to come back with a positive test for sarin?
The machine used to detect risky substances at the mail processing center reportedly identified sarin on the package on two out of three tests, but it is unclear whether they were false positives.
On Monday evening at around 6 p.m., the San Mateo County Hazardous Materials and the California National Guard's Civil Support Team entered the Facebook mail facility to test the package.
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That was ordered after a bag containing mail "tested positive" for the agent, the report said.
A colorless, odorless liquid, sarin is an extremely potent nerve agent, generally considered a weapon of mass destruction.
It can evaporate into gas and spread easily into the environment if fired off and, while there are antidotes for it, they must be administered within minutes in order for them to be effective. (CNN) The building was evacuated after the find.
In 1995 more than 5000 people fell ill following a sarin attack by the Aum Shinrikyo cult, who targeted trains heading to the Tokyo's government district in Japan.
Fire officials and the Federal Bureau of Investigation were called in to investigate the situation.
Facebook did not immediately respond to Deadline's request for comment.