Emergency crews recovered two bodies from the wreckage. The pilot was presumed dead and one worker in the building had been missing. Austin Fire Department Battalion Chief Palmer Buck declined to discuss the identities of those found, but said Thursday night that authorities had "accounted for everybody."
The FBI tentatively identified the pilot as A. Joseph Stack III, 53. Law enforcement officials, speaking on condition of anonymity because the investigation was still going on, said that before taking off, Stack apparently set fire to his house and posted a long anti-government screed on the Web. It was dated Thursday 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."
The pilot took off in a four-seat, single engine Piper PA-28 from an airport in Georgetown, about 30 miles from Austin, without filing a flight plan. He flew low over the Austin skyline before plowing into the side of the hulking, seven-story, black-glass building just before 10 a.m. with a thunderous explosion that instantly stirred memories of Sept. 11.
Flames shot from the building, windows exploded, a huge pillar of black smoke rose over the city, and terrified workers rushed to get out.
The Pentagon scrambled two F-16 fighter jets from Houston to patrol the skies over the burning building before it became clear that it was the act of a lone pilot, and President Barack Obama was briefed.
"It felt like a bomb blew off," said Peggy Walker, an IRS revenue officer who was sitting at her desk. "The ceiling caved in and windows blew in. We got up and ran."
At least 13 people were injured, with two reported in critical condition. About 190 IRS employees work in the building.
Gerry Cullen was eating breakfast at a restaurant across the street when the plane struck the building and "vanished in a fireball."
Matt Farney, who was in the parking lot of a nearby Home Depot, said he saw a low-flying plane near some apartments 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 a half-mile from the crash, Michelle Santibanez felt the vibrations and ran to the windows, where she and her co-workers witnessed a scene that reminded them of 9/11.
"It was the same kind of scenario, with window panels falling out and desks falling out and paperwork flying," said Santibanez, an accountant.
The building, in a heavily congested section of Austin, was still smoldering six hours later, with the worst of the damage on the second and third floors.
The entire outside of the second floor was gone on the side of the building where the plane hit. Support beams were bent inward. Venetian blinds dangled from blown-out windows, and large sections of the exterior were blackened with soot. It was not immediately clear if any tax records were destroyed.
Andrew Jacobson, an IRS revenue officer who was on the second floor when the plane hit with a "big whoomp" and then a second explosion, said about six people couldn't use the stairwell because of smoke and debris. He found a metal bar to break a window so the group could crawl out onto a concrete ledge, where they were rescued by firefighters. His bloody hands were bandaged.
Austin Police Chief Art Acevedo said "heroic actions" by federal employees may explain why the death toll was so low. The FBI was investigating. The National Transportation Safety Board sent an investigator as well.
Rep. Michael McCaul, a Republican from Austin on the Homeland Security Committee, said the panel will take up the issue of how to better protect buildings from attacks with planes.
In the long, rambling, self-described "rant" that Stack apparently posted on the Internet, he began: "If you're reading this, you're no doubt asking yourself, `Why did this have to happen?"'
He recounted his financial reverses, his difficulty finding work in Austin, and at least two clashes with the IRS, one of them after he filed no return because, he said, he had no income, the other after he failed to report his wife Sheryl's income.
He railed against politicians, the Catholic Church, the "unthinkable atrocities" committed by big business, and the government bailouts that followed. He said he slowly came to the conclusion that "violence not only is the answer, it is the only answer."
"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," he wrote.
According to California state records, Stack had a troubled business history, twice starting software companies in California that ultimately were suspended by the state's tax board, one in 2000, the other in 2004. Also, his first wife filed for bankruptcy in 1999, listing a debt to the IRS of nearly $126,000.
The blaze at Stack's home, a red-brick house on a tree-lined street in a middle-class neighborhood six miles from the crash site, caved in the roof and blew out the windows.
Elbert Hutchins, who lives one house away, said the house caught fire about 9:15 a.m. He said a woman and her 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!"'
Red Cross spokeswoman Marty McKellips said the agency was treating two people who live in the house and that the family had no comment Thursday. McKellips said the family would "give information and answer questions" on Friday.
Thursday was not the first time a tax protester went after an Austin IRS building. In 1995, Charles Ray Polk plotted to bomb the IRS Austin Service Center. He was released from prison in October of last year.
The tax protest movement has a long history in the U.S. and was a strong component of anti-government sentiments that surged during the 1990s. Anti-tax protesters typically believe that they do not have to pay income taxes. Some have been convicted in recent years for targeting IRS officials for harassment and even murder.
Associated Press writers April Castro and Jay Root in Austin; Michelle Roberts in Georgetown; Linda Stewart Ball, Danny Robbins, Jeff Carlton and John McFarland in Dallas; Devlin Barrett, Lolita C. Baldor, Eileen Sullivan and Joan Lowy in Washington; and Melanie Coffee and Barbara Rodriguez in Chicago contributed to this report, along with the AP News Research Center.