A Facebook mail processing warehouse has been evacuated as Silicon Valley officials try to determine what caused a bag of mail to test positive for the nerve agent sarin.

Menlo Park Fire Marshal Jon Johnston said Monday that incoming mail going through routine processing tested positive for sarin. He said authorities are checking to see if the test was a false positive or something else.

Johnston said nobody is showing symptoms of exposure and that the building is under quarantine. Johnston did not know how many people were evacuated.

The federal Centers for Disease Control and Prevention says symptoms to sarin vapor exposure will likely appear within seconds. The CDC says people who are mildly exposed usually recover completely.

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(AP)