A false alarm at Facebook's offices prompted the evacuation of four buildings over the potential presence of deadly poison sarin.

The company has since confirmed that there is no trace of the substance.

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A detection of the deadly poison sarin at Facebook's offices in Menlo Park, California, has turned out to be a false alarm.

On Monday, a routine test at a mail facility for the company tested positive for the substance, prompting the evacuation of four buildings, FBI involvement, and the quarantine of the suspect mailbag.

In a statement issued early Tuesday, Facebook spokesperson Anthony Harrison said: "At approximately 11:00 AM PDT on July 1, 2019, mail delivered to one of our mail rooms in Menlo Park was deemed suspicious. As part of Facebook's routine mail security screening process, we identified a potentially dangerous substance.

"Out of an abundance of caution, we evacuated four nearby buildings and began a thorough investigation in coordination with local authorities. Authorities have confirmed test results were negative for any potentially dangerous substance and the buildings have been cleared for repopulation. Our rigorous security and safety procedures worked as intended to limit exposure and keep our people safe."

A number of people who work at the facility were being monitored, and no one showed any symptoms of exposure, according to Menlo Park Fire Department officials.

Sarin is a man-made nerve agent used in chemical warfare. Exposure can cause loss of consciousness, convulsions, paralysis, respiratory failure, and death, according to the Centers for Disease Control and Prevention.