The home was destroyed, but officials have not linked the two incidents.
Stack apparently left a lengthy anti-government manifesto on a social media web site, the Austin American-Statesman reported. Read the manifesto.
The pilot of the plane that crashed did not file a flight plan or, as far as FAA officials know at this time, have any other contact with the agency.
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. Officials say they are confident the crash was an isolated incident and "there is no cause for concern" about terrorism.
Assistant Austin Fire Chief Harry Evans said two people have also been taken to a hospital. Their conditions were not immediately known.
Thick black and gray smoke was billowing out of the second and third stories of the building Thursday as fire crews using ladder trucks and hoses battled the fire. Dozens of windows were blown out of the hulking black building, and vehicles traveling on a nearby highway paused to look.
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.
Matt Farney, 39, who was in the parking lot of a nearby Home Depot, said he saw a low-flying private 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, adding that the plane dipped down. "It was a ball of flames that was high or higher than the apartments. It was surreal. It was insane. ... It didn't look like he was out of control or anything."
Sitting at her desk about a half-mile from the crash, Michelle Santibanez said she felt vibrations. She and her co-workers ran to the windows, where they saw a scene that reminded them of the 2001 terrorist 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.
Fire crews were inside the building battling the blaze and looking for survivors, Evans said.
Federal Aviation Administration spokesman Lynn Lunsford said the agency was investigating but had no immediate information on the type of plane or how many people were on board.
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Associated Press writers April Castro and Jay Root in Austin and Devlin Barrett in Washington contributed to this report.
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February 18, 2010 05:38 PM EST
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