The FBI responded to Facebook’s mailing facility in Menlo Park after an unknown substance was detected inside a suspicious package. The agency’s San Francisco branch confirmed to The Daily Beast it was responding to the scene at the request of a San Mateo hazmat team. Menlo Park Fire Chief Harold Schapelhouman reportedly said tests on a package revealed “mixed results,” so the package was “isolated”—but no chemicals were released in the facility. Facebook spokesperson Anthony Harrison said in a statement that one package delivered to the facility on Monday morning was deemed “suspicious” and the company subsequently evacuated four buildings. Harrison also said the substance has yet to be identified, but said three buildings were cleared for repopulation.