FILE - The entrance sign to Facebook headquarters is seen through two moving buses in Menlo Park, California, Oct. 10, 2018.
FILE - A sign at Facebook headquarters in Menlo Park, California, July 2013.

Facebook evacuated a mail facility near its Silicon Valley company headquarters Monday after a package tested positive for the nerve agent sarin. 

The company said two people were being evaluated for possible exposure to the toxic compound. It said the two have not shown any symptoms of exposure, but are being tested because they came into contact with the suspicious package. 

Authorities ordered the mail facility, which included four buildings in Menlo Park, under quarantine and conducted tests on the area. They later cleared three buildings and allowed employees working in them to return.

Facebook spokesman Anthony Harrison said authorities have not yet identified the substance found in the suspicious package and said the company is cooperating with police.

Local authorities say the mail facility routinely uses machines to test incoming mail for toxins and say one package was flagged for sarin. They say it is possible that the machine gave a false positive. 

Sarin disrupts the nervous system and has been used as a chemical weapon. A drop of the substance on the skin can cause muscle twitching while larger doses can result in paralysis and death.