UPDATED, 7/2: Facebook evacuated four buildings at its Menlo Park, Calif., campus Monday after equipment at the company’s mail-processing facility detected a package that might have contained sarin, a potentially lethal nerve agent.

According to Facebook, tests came back negative for sarin and other toxic material.

“Authorities have confirmed test results were negative for any potentially dangerous substance and the buildings have been cleared for repopulation,” Facebook director of corporate media relations Anthony Harrison said in a statement early Tuesday.

A package flagged as “suspicious” was delivered at around 11 a.m. PT on July 1 to one of Facebook’s mail rooms in Menlo Park. “Out of an abundance of caution, we evacuated four nearby buildings and began a thorough investigation in coordination with local authorities. Our rigorous security and safety procedures worked as intended to limit exposure and keep our people safe,” Harrison said.

As part of standard procedures, Facebook tests packages sent through its mail facilities for dangerous substances.

Facebook’s initial detection of the potential presence of sarin in the package was first reported by NBC Bay Area. Officials said two individuals suspected of exposure to the nerve agent did not exhibit symptoms of sarin poisoning, Reuters reported.

Local authorities called in the FBI and the National Guard after the buildings were evacuated. The Bay Area’s ABC 7 reported that after first responders were alerted to the potential presence of sarin, a secondary open-air test came back negative for the chemical.

Sarin is a man-made toxin described as a “chemical warfare agent classified as a nerve agent,” according to the Centers for Disease Control and Prevention. Exposure to large amounts of sarin can lead to respiratory failure possibly leading to death.