A Facebook postal facility near the company's California headquarters was evacuated last night after a routine check found mail possibly containing the nerve agent sarin.
Authorities put the Menlo Park site under quarantine as they conducted additional testing.
Four of the social media company's buildings were evacuated and two people were checked for possible exposure to the compound that attacks the nervous system and can be fatal.
But exhaustive testing by fire and hazardous material teams found no toxic material.
"There is no sarin," said Jon Johnston, fire marshal for the city of Menlo Park, referring to the package that had erroneously tested positive on Monday morning.
Facebook routinely checks all packages and had initiated a standard safety protocol, Johnston added, saying teams worked into the early hours of Tuesday to clear the scene.
Federal Bureau of Investigation (FBI) agents also assisted the investigation, Facebook said.
With 2.3 billion monthly active users and more than $55 billion (£43 billion) in revenue in 2018, Facebook is massively popular around the world.
But it also faces criticism for its control of personal information and has been subject to cyber attacks.
In December, it had a bomb threat at its main campus in Menlo Park that forced the evacuation of several buildings. No bomb was found.
Sarin is a chemical warfare agent that is a clear, colourless, odourless and tasteless liquid. It can evaporate into the environment, prompting symptoms within seconds.
A drop of sarin on skin can cause sweating and muscle twitching, and exposure to large doses can result in paralysis and respiratory failure leading to death.
