A suspicious package mailed to Facebook headquarters in Menlo Park was flagged by a screening machine as possibly containing sarin gas. No employees were reportedly showing signs of exposure, but multiple buildings in the Facebook complex were evacuated out of caution.

Only one building remained evacuated as of noon, according to NBC Bay Area. SFGate reported via some early reports from the fire district that two Facebook employees might have been exposed to the deadly nerve agent, but they were not showing any symptoms.

Facebook reportedly puts all of its mail through a screening machine that detects explosives and poisons like sarin. The mail sorting facility is in an industrial park at 1195 Hamilton Court in Menlo Park, south of the company's two adjacent main campus, the "old" campus referred to by employees as Facebook Classic, and the newer Buildings 20 and 26, designed by Frank Gehry.

A hazmat team was on the scene investigating, as CBS SF reports, and it remains unclear whether the deadly gas was actually present. Authorities say that two tests came up positive for sarin, and one test was negative.

Sarin is deadly in liquid or vapor form, and is a man-made nerve agent that is both odorless and colorless.