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Facebook said early Tuesday that a "suspicious" package, which had arrived at one of the social media giant's mail rooms in Menlo Park on Monday, didn't contain a toxic chemical.

The false alarm prompted the company to evacuate four nearby buildings on Monday. Authorities, including the Menlo Park Fire District, the National Guard and the FBI, sent teams to investigate what was in the package.

NBC News Bay Area and KRON 4 initially reported the substance may have been sarin, a dangerous nerve agent, after a machine at the facility warned of a possible toxic substance.

Facebook spokesman Anthony Harrison said the company had been screening the mail in its facility when employees "identified a potentially dangerous substance" around 11 a.m. PT on Monday.

Menlo Park Fire District

"Authorities have confirmed test results were negative for any potentially dangerous substance and the buildings have been cleared for repopulation," he said. "Our rigorous security and safety procedures worked as intended to limit exposure and keep our people safe."

Cameron Polan, a spokeswoman for the FBI, said in a statement on Tuesday that the agency and other law enforcement groups tested the suspect mail and determined it to be "non-hazardous."

On Monday morning, the San Mateo County Public Safety Communications Center received a call from someone who said a package "was suspected to contain a chemical agent inside," the Menlo Park Fire District said in a news release. The caller added that two people may have been exposed and said the substance may have been sarin.

Initial reports that two people were exposed and needed medical attention were false.

Once the firefighters arrived at Facebook's mail-and-package facility on Hamilton Court, Capt. Roy Trester told the fire district that "we have a reported positive hit for sarin, there is no medical component and I am requesting a full Hazardous Materials Response." About a dozen Facebook employees were evacuated from nearby buildings.

On Monday evening at around 6 p.m., the San Mateo County Hazardous Materials and the California National Guard's Civil Support Team entered the Facebook mail facility to test the package. The crews went into the building again after 10:30 p.m. and secured the package before removing it from the mail facility.

The incident isn't the first time a tech company has had to deal with a security issue. In April 2018, a woman shot three people at YouTube's headquarters before turning the gun on herself. In that case, the shooter appeared to have been angry at how YouTube had handled her channels and videos. In December, Facebook evacuated its headquarters after receiving an anonymous bomb threat that turned out to be a false alarm.

Menlo Park Fire Chief Harold Schapelhouman said in a statement that the suspicious package at Facebook's mail facility "did not present a threat to the broader community."

"Out of an abundance of caution, the fire crews and other agencies on-scene took a very methodical and careful approach to how this event was handled," he said.

First published July 1 at 2:05 p.m. PT.

Update, July 2: Adds that tests proved negative, plus more detail from the FBI and Menlo Park Fire District.