The building contained 200 federal tax employees at the time of the crash that started a raging fire and sent workers fleeing for their lives.
Thirteen people were treated in hospital and two remained in critical condition Thursday evening, authorities said.
The pilot took off in a single-engine Piper PA-28 from an airport in Georgetown, about 30 miles from Austin. The FBI tentatively identified the pilot as Andrew Joseph Stack III, 53.
The Georgetown, Texas airport was evacuated and a bomb squad arrived to do the scene to check Stack's car following the incident. Federal authorities declared the airport all clear and found no bomb the vehicle.
Planes scheduled to land at Georgetown Airport were being re-routed to Austin Bergstrom Airport in Austin while the vehicle was searched.
Law enforcement officials said that before taking off, Stack apparently set fire to his house and posted a long antigovernment screed on the Web. It was dated yesterday and signed "Joe Stack (1956-2010)."
In it, the author cited run-ins he had with the IRS and ranted about the tax agency, government bailouts, and corporate America's "thugs and plunderers."
"I have had all I can stand," he wrote, adding: "I choose not to keep looking over my shoulder at 'big brother' while he strips my carcass."
Andrew Joseph Stack suspected in Austin plane crash Washington Post
Joe Stack: Antitax 'terrorist' or solo IRS-hater? Christian Science Monitor
Joe Stack Plane Crash Austin Aftermath: 13 Injured, Two Critically CBS News
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