Facebook buildings in San Francisco, Calif., were evacuated Monday after a suspicious package tested positive for sarin in the Facebook mailing facility.

All mail sent to the facility is pre-screened by machines to detect hazardous substances and employees were alerted before the package was handled internally, according to NBC San Francisco. Four buildings were initially evacuated, but three have since been deemed safe and resumed operations. Initial reports from firefighter officials said that two employees may have been exposed, but they have since corrected the record to say that no employees having been exposed at this time.