A US law official identified the pilot as Joseph Stack and said investigators were looking at an anti-government message on the web linked to him. The website outlines problems with the IRS and says violence "is the only answer."
Federal law enforcement officials have said they were investigating whether the pilot crashed on purpose in an effort to blow up IRS offices. The website featured a long note dated Thursday denouncing the government and the IRS in particular and cited the Austin man's problems with the agency.
All the officials spoke on condition of anonymity because the investigation is continuing.
At least one person who worked in the building was unaccounted for and two people were hospitalised, said Austin Fire Department Division Chief Dawn Clopton. She did not have any information about the pilot.
Flames shot out of the building, windows exploded and workers scrambled to safety after the blast. Thick smoke billowed out of the second and third stories hours later as fire crews battled the blaze.
"It felt like a bomb blew off," said Peggy Walker, an IRS revenue officer who was sitting at her desk in the building when the plane crashed. "The ceiling caved in and windows blew in. We got up and ran."
Federal Aviation Administration spokesman Lynn Lunsford initially said the plane was identified as a Cirrus SR22, but later said it might be a Piper Cherokee.
"It's so destroyed that it's hard to identify," Lunsford said.
He said FAA has confirmed that the plane that took off from an airport in Georgetown, Texas, and that the pilot didn't file a flight plan.
In a neighbourhood about 10km from the crash site, a home listed as belonging to Stack was on fire earlier on Thursday. Authorities in Austin would not comment on the house fire.
