A piece of mail flagged as suspicious at a Facebook mailing facility in Menlo Park, California, on Monday has been determined to be non-hazardous, authorities and the company said Tuesday.
After a machine that can detect dangerous substances alerted employees that the package might contain the chemical nerve agent sarin Monday morning, emergency hazmat crews spent several hours at the facility, NBC reported.
No employees were ever exposed to a substance, according to the fire district, according to NBC.
"The FBI and our law enforcement partners thoroughly tested the items in question and determined them to be non-hazardous," a spokesperson for the Federal Bureau of Investigation told CNBC. "This collaborative effort took time, but ensured a most careful and methodical testing was taking place to protect the public."
Facebook also tweeted about the event Tuesday, saying that the company evacuated nearby buildings and began an investigation with nearby authorities.
"Test results were negative, no dangerous substance was identified and the buildings have returned to normal operation," Facebook's tweet said.
Sarin is a "human-made chemical warfare agent classified as a nerve agent," according to the Centers for Disease Control and Prevention.