AUSTIN, Texas -- A software engineer furious with the Internal Revenue Service launched a suicide attack on the agency Thursday by crashing his small plane into an office building containing about 190 IRS employees, setting off a raging fire that sent workers running for their lives.

At least one person in the building was missing.

The FBI tentatively identified the pilot as Joseph A. Stack, 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 9/11.

Flames shot from the building, windows exploded, a huge pillar of black smoke rose over the city and terrified workers rushed to get out.

"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."

Stack was presumed dead. Emergency crews found a body in the building Thursday night, but Police Chief Art Acevedo declined to say whether it was the pilot.

At least 13 people were injured, with two reported in critical condition.

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 yet known if any tax records were destroyed.

In the 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 didn't 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."