It happened around 7:30 Monday morning at the IRS office on Butler and Winery in Southeast Fresno.
Employees were opening mail and found two different envelopes full of white powder. The fire department established a command center and their hazmat team determined the powder is caustic.
"When it gets on your skin, if your skin was moist or if you had water it would react it could cause burns or skin irritation," Sean Johnson of the Fresno Fire Department said,
The area was isolated and the fire department says none of the ten thousand employees who work there was injured.
The Treasury Department is now investigating.