The IRS website said an office of its EP Team Audit Programme, which examines employee benefit plans, was located in the building.
Peggy Walker, an IRS revenue officer who works in the building, said she was sitting at her desk when the plane crashed.
"It felt like a bomb blew off. The ceiling caved in and windows blew in. We got up and ran," she said.
Lynn Lunsford, a federal aviation administration spokesman, said the agency was investigating but had no immediate information on the type of plane or how many people were on board.
Peter Knudson, the national transportation safety board spokesman, said an investigator from the board's Dallas office had been dispatched to the scene of the accident to start an investigation.
Federal law enforcement officials said they were investigating whether the pilot crashed on purpose in an effort to blow up IRS offices.
The pilot, found dead at the crash site, in his note on the website listed out his problems with the IRS.