A Facebook mail processing warehouse was evacuated as Silicon Valley officials work to determine what caused mail to test positive for the nerve agent sarin.
Menlo Park Fire Marshal Jon Johnston said incoming mail undergoing routine processing by machine tested positive for sarin, but there were no reports of injuries.
"Right now we don’t have anybody that has any symptoms," he said.
"We’re just doing verification," he added.
Facebook spokesman Anthony Harrison said four buildings were evacuated and three have been cleared for people to go back in.
The suspicious package was delivered to one of the company’s mail rooms, he said.
"Authorities have not yet identified the substance found," Mr Harrison wrote.
The federal Centres for Disease Control and Prevention says sarin is a chemical warfare agent that is a clear, colourless, odourless and tasteless liquid.
It can evaporate into the environment, prompting symptoms within seconds.
A drop of sarin on skin can cause sweating and muscle twitching, and exposure to large doses can result in paralysis and respiratory failure leading to death.
The CDC said people who are mildly exposed usually recover completely.