Who mailed two envelopes containing a white powder to a Fresno Internal Revenue Service office Monday?
That's what federal agents would now like to find out.
The Fresno Fire Hazmat team rushed to the building near Butler and Helm in Southeast Fresno early Monday.
Hazmat crews evacuated the room where the envelopes were found, safely removed the powder and checked out the 20 people in the room at the time.
No one was hurt.
Investigators say the white powder was a "caustic substance" capable of causing burns and skin irritation.
The U.S. Treasury Department is now investigating who may have sent it.
This isn't the first time the Fresno IRS office has been the target of malicious mail.
Last February, employees opened a package in the downtown mailroom to find a box full of poop.
The Fresno Fire Department's Hazmat team came and evacuated the building and called in the bomb squad.
No one was hurt in that incident either.