A U.S. 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 Web site outlines problems with the IRS and says violence "is the only answer."
Federal law enforcement officials have said they were investigating whether the pilot, who is presumed to have died in the crash, slammed into the Austin building on purpose in an effort to blow up IRS offices. All the officials spoke on condition of anonymity because the investigation was ongoing.
"Violence not only is the answer, it is the only answer," the long note on Stack's Web site reads, citing past problems with the tax-collecting agency.
"I saw it written once that the definition of insanity is repeating the same process over and over and expecting the outcome to suddenly be different. I am finally ready to stop this insanity. Well, Mr. Big Brother IRS man, let's try something different; take my pound of flesh and sleep well," the note, dated Thursday, reads.
At least one person who worked in the building was unaccounted for and two people were hospitalized, said Austin Fire Department Division Chief Dawn Clopton. She did not have any information about the pilot. About 190 IRS employees work in the building, and IRS spokesman Richard C. Sanford the agency is trying to account for all of its workers.
After the low-flying plane crashed into the building, flames shot out, windows exploded and workers scrambled to safety. 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."
In a neighborhood about six miles from the crash site, a home listed as belonging to Stack was on fire earlier Thursday. Two law enforcement officials said Stack had apparently set fire to his home before the suicidal plane flight.
Elbert Hutchins, who lives one house away from the house on a quiet, tree-lined middle class neighborhood, said the house caught fire about 9:15 a.m. He said a woman and her teenage daughter drove up to the house before firefighters arrived.
"They both were very, very distraught," said Hutchins, a retiree who said he didn't know the family well. "'That's our house!' they cried 'That's our house!' "
Federal Aviation Administration spokesman Lynn Lunsford said the agency confirmed the plane took off from an airport in Georgetown, Texas, and the pilot didn't file a flight plan. Lunsford said initially the plane was identified as a Cirrus SR22 but later said it might be a Piper Cherokee.
Gerry Cullen, 66, was eating breakfast a restaurant across the street when the plane struck the building.
"The airplane hit and vanished in a fireball," said Cullen, a former flight instructor.
Matt Farney, 39, who was in the parking lot of a nearby Home Depot, said he saw a low-flying small plane near some apartments and the office building just before it crashed.
"I figured he was going to buzz the apartments or he was showing off," Farney said. "It was insane. ... It didn't look like he was out of control or anything."
Sitting at her desk in another building about a half-mile from the crash, Michelle Santibanez said she felt vibrations after the crash. She and her co-workers ran to the windows, where they saw a scene that reminded them of the 9/11 attacks, she said.
"It was the same kind of scenario with window panels falling out and desks falling out and paperwork flying," said Santibanez, an accountant.
National Transportation Safety Board spokesman Peter Knudson said an investigator from the board's Dallas office has been dispatched to the scene of the accident to start an investigation. The FAA and NTSB officials said they had no information on whether the crash was intentional. The White House also said President Barack Obama was briefed about the crash.
As a precaution, the Colorado-based North American Aerospace Defense Command launched two F-16 aircraft from Houston's Ellington Field, and is conducting an air patrol over the crash area.