Several buildings at Facebook’s Menlo Park campus have been evacuated after a machine at the company’s mailing facility alerted employees that a package may contain sarin, a potentially deadly nerve agent, as first reported by NBC Bay Area. According to that report, no one has been exposed to the substance, whatever it may be.

“We evacuated four buildings and are conducting a thorough investigation in coordination with local authorities,” Anthony Harrison, a Facebook spokesperson told The Verge. “As of now, three buildings have been cleared for repopulation. The safety of our employees is our top priority and we will share additional information when it is available.”

Facebook routinely tests packages sent through its mailing facility for dangerous chemicals and substances, according to Business Insider. The mailbag that set off alarms is currently under quarantine. No one yet knows if the substance actually is sarin.

At 11 a.m. PDT, the package was delivered to Facebook and the company began evacuating some of the Menlo Park campus buildings. As of right now, no employees have exhibited symptoms of sarin poisoning, but two are currently being monitored by authorities, officials told Business Insider.