What just happened? A Facebook mailing facility in Menlo Park, California, was evacuated earlier today due to the detection of traces of the deadly nerve agent sarin. Sarin is an extremely toxic compound. It is odorless, colorless, and tasteless, making it very hard to detect by humans.

According to the Los Angeles Times, at around 11 am Monday a package scanner in a Facebook mailroom detected traces of sarin prompting an evacuation of four buildings. As of this writing, only one of the buildings remains locked down. Officials from the Federal Bureau of Investigation have arrived on the scene and have opened an investigation.

Facebook spokesperson Anthony Harrison issued a statement to KABC-TV.

"At 11:00 AM PDT this morning, a package delivered to one of our mailrooms was deemed suspicious. We evacuated four buildings and are conducting a thorough investigation in coordination with local authorities. Authorities have not yet identified the substance found. As of now, three of the evacuated 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."

It was reported earlier that two employees had been exposed to the toxin, but Fire Chief Harold Schapelhouman told the LA Times that no one had come in contact with the chemical. An initial open-air test showed no signs of the sarin compound. It is possible that the detection was a false positive.

"Sometimes machines make mistakes," Schapelhouman said. "This is a standard package-handling call we're dealing with."

The FBI was dispatched to the scene at 1 pm, and according to ABC7's Chris Nguyen, the National Guard has been notified as well.

Investigators will open the package to see if it tests positive for the nerve agent.

Update 7/2/19: Members of the Menlo Park Fire District HazMat Team entered the closed off building at around 6 pm Monday night and tested the isolated package. Testing confirmed that there were no potentially dangerous substances in the package. The team exited the building at 6:45 pm.

Later Monday night, the team performed a second sweep of the building using "more sophisticated detection equipment." They removed the package from the facility in a containment device and tested it again off-site. Results were again negative. The building was cleared for occupation this morning.

The battalion chief told ABC7 that this is the second time that the Facebook facilities detector has triggered a false positive. Facebook has not issued further comment on the incident.