Menlo Park Fire Marshal Jon Johnston told The Associated Press that incoming mail going through routine processing tested positive for sarin. The Menlo Park Fire Protection District said it was called to the area shortly before 11 a.m.
Johnston said investigators were checking to see if the test was a false positive or something else. He added that no one was showing signs of exposure. No other injuries were reported.
Facebook spokesman Anthony Harrison confirmed the buildings were evacuated and said the company was "conducting a thorough investigation in coordination with local authorities." Harrison added that authorities had not yet identified the substance, but noted that three of the evacuated buildings had been given the all-clear.
"The safety of our employees is our top priority and we will share additional information when it is available," he said.
It was not immediately clear how many people were forced to leave the affected buildings. The facility is located a short distance from Facebook's corporate headquarters.
The federal Centers for Disease Control and Prevention (CDC) has reported that symptoms to sarin vapor exposure likely would appear within seconds. The CDC added that people who are mildly exposed usually recover completely.
The Associated Press contributed to this report.