Hazmat crews are investigating the possibility of a package sent to a Facebook mail facility in California containing the nerve agent sarin.
Initial reports quoting a fire official said two people "were possibly exposed to sarin" at the facility in Menlo Park, but were "hot exhibiting any signs or symptoms".
Sarin, a toxic compound that disrupts the nervous system, can be used as a chemical weapon.
Developed in Germany in 1938 as a pesticide, sarin is a colourless, odourless and tasteless liquid that can evaporate rapidly into gas.
Low exposure can cause sweating and muscle-twitching, while larger doses - which have been used in chemical warfare - can lead to death.
Early Hazmat test results from the mail bag at the Facebook facility that originally sparked the alert proved inconclusive.
It had tested positive for sarin during a routine mail check carried out by Facebook.
Menlo Park Fire Marshall Jon Johnston said the result could have been a false positive, adding that further testing would need to be carried out.
In a statement, Facebook said it evacuated four buildings after the "suspicious package" was received on Monday.
Three of the buildings were later cleared, while one remained under evacuation amid testing.
"The safety of our employees is our top priority and we will share additional information when it is available," Facebook spokesperson Anthony Harrison said.