AUSTIN, Tex. — Leaving behind a rant against the government, big business and particularly the tax system, a computer engineer smashed a small aircraft into an office building where nearly 200 employees of the Internal Revenue Service were starting their workday Thursday morning, the authorities said.

The pilot, identified as Andrew Joseph Stack III, 53, of north Austin, apparently died in the crash, and one other person was unaccounted for. Late Thursday, two bodies were pulled from the site, though the authorities would not discuss the identities of those found, The Associated Press reported. Two serious injuries were also reported in the crash and subsequent fire, which initially inspired fears of a terrorist attack and drew nationwide attention.

But in place of the typical portrait of a terrorist driven by ideology, Mr. Stack was described as generally easygoing, a talented amateur musician with marital troubles and a maddening grudge against the tax authorities.

“I knew Joe had a hang-up with the I.R.S. on account of them breaking him, taking his savings away,” said Jack Cook, the stepfather of Mr. Stack’s wife, in a telephone interview from his home in Oklahoma. “And that’s undoubtedly the reason he flew the airplane against that building. Not to kill people, but just to damage the I.R.S.”Photo

Within hours of the crash, before the death or even the identity of the pilot had been confirmed, officials ruled out any connection to terrorist groups or causes.

“The main thing I want to put out there is that this is an isolated incident here; there is no cause for alarm,” said the Austin police chief, Art Acevedo, in a televised news conference at midday. Asked how he could be sure, Mr. Acevedo said, “You have to take my word at it, don’t you?”

As the Department of Homeland Security opened an investigation and President Obama received a briefing from his counterterrorism adviser, John O. Brennan, federal officials emphasized the same message, describing the case as a criminal inquiry.

Since the attacks of Sept. 11, 2001, the notion of terrorists using small airplanes to crash into buildings has raised a special sort of public anxiety. That was the initial reaction in 2006, when a New York Yankees pitcher and his flight instructor died in a crash in Manhattan. On Thursday the North American Aerospace Defense Command sent two F-16 aircraft to patrol the area before it was determined that the crash was the work of one man.

Mr. Stack’s aircraft, a single-engine fixed-wing Piper PA-28-236 registered in California, took off from Georgetown Municipal Airport, about 25 miles north of Austin, at 9:40 a.m., the Federal Aviation Administration said.

At 9:56, the plane tore through a seven-story office building at 9430 Research Boulevard, about seven miles northwest of the State Capitol, local authorities said. Flames and smoke engulfed the building, sending big black burned panels to the ground. Emergency medical officials said two men were injured, both in the fire. One was transported to a burn unit in San Antonio. A third office worker was described only as unaccounted for.

Aside from the I.R.S., private organizations including an education center affiliated with St. Edward’s University maintain offices in the building, according to address records. The local office of the Federal Bureau of Investigation is in a separate part of the complex.

“We can confirm that the building that the plane hit this morning includes I.R.S. offices,” said Terry L. Lemons, a spokesman for the agency. “We have about 190 employees that work at those offices. We’re still in the process of accounting for everyone.”

In a six-page statement signed “Joe Stack (1956-2010)” and posted on a Web site connected to Mr. Stack’s wife, the author singled out the tax agency as a source of suicidal rage, concluding, “Well, Mr. Big Brother I.R.S. man, let’s try something different, take my pound of flesh and sleep well.”

Though profane at points, the statement articulated grievances with specific sections of the tax code, corporations, politicians and a local accountant. It appeared to have been written with some deliberation. At one point, the verbs “left” and “abandoned” appear side by side, seemingly an editing choice never settled.

From relatives, friends and neighbors, a portrait emerged of Mr. Stack as a man pushed over the brink by retirement dreams deferred by a long series of financial setbacks.

By the account of Mr. Cook, Mr. Stack was raised in an orphanage in Hershey, Pa., with a brother and sister, leaving the orphanage after high school to attend college. He worked as a software engineer in California, learned to fly and played guitar and piano for recreation. He moved to Austin, playing with a band and at informal gatherings.

Mr. Stack met Mr. Cook’s stepdaughter, the former Sheryl Housh, through musician friends in Austin. After eight months of friendship, they dated and married about three years ago. Both had been previously married.

Mrs. Stack, 50, listed in records at the University of Texas as a graduate student in music performance, brought her own back story to the marriage, having spent several years in the sway of a religious cult before her parents orchestrated a rescue.

On visits to Oklahoma, Mr. Stack took his new in-laws up in his plane. He never spoke of his troubles with the I.R.S., though his wife related them. The family assembled in Austin at Christmas, and Mr. Stack seemed fine, Mr. Cook said.

But in recent weeks Mrs. Stack complained to her parents of an increasingly frightening anger in her husband, straining the marriage, Mr. Cook said. On Wednesday night, Mrs. Stack took her 12-year-old daughter, Margaux, to a hotel to get away from her husband.

They returned on Thursday morning to find their house ablaze, their belongings destroyed. Officials said the house fire was deliberately set, casting Mr. Stack as the primary suspect. But by that point he was gone, airborne.

“This is a shock to me that he would do something like this,” Mr. Cook said. “But you get your anger up, you do it.”