A pilot furious with the Internal Revenue Service crashed his small plane into an Austin, Texas, office building where nearly 200 federal tax employees work on Thursday, igniting a raging fire that sent massive plumes of thick, black smoke rising from the seven-story structure.

Austin Police Chief Art Acevedo said the incident was a single act by a sole individual, who appeared to be targeting the federal building. He refused to classify it as terrorism.

"I call it a cowardly, criminal act and there was no excuse for it," Acevedo said at a news conference.

The FBI identified the pliot as Joseph Stack, a 53-year-old software engineer. Stack was confirmed dead, but his body has not yet been recovered.

At least one person who worked in the building was unaccounted for and two people were hospitalized, thirteen others were treated and released said Austin Fire Department Division Chief Dawn Clopton.

Emergency crews found two bodies in the building late Thursday evening, but wouldn't identify them.

Texas Republican Congressman Michael McCaul told reported the incident was, "not tied to overseas terror organizations."

A U.S. law official said investigators were looking at a lengthy, anti-government "manifesto" Stack is believed to have written on his Web site. The message outlines problems with the IRS and says violence "is the only answer."

Click here to read the "manifesto" that was published on Stack's Web site.

About 190 IRS employees work at 9420 Research Boulevard, the building that Stack crashed into. IRS spokesman Richard C. Sanford said the agency is trying to account for all of its workers.

IRS Agent William Winnie said he was on the third floor of the building when he saw a light-colored, single engine plane coming toward the building, TheStatesman.com reported.

“It looked like it was coming right in my window,” Winnie said, according to the Web site.

SLIDESHOW: Small Plane Crashes Into Austin Office Building

He said the plane veered down and smashed into the lower floors. “I didn’t lose my footing, but it was enough to knock people who were sitting to the floor,” he said.

In what appears to have been his suicide note, Stack is believed to have written:

"If you’re reading this, you’re no doubt asking yourself, “Why did this have to happen?' The simple truth is that it is complicated and has been coming for a long time...

"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," the note, dated Thursday, reads.

IRS Commissioner Doug Shulman said he was shocked by the "tragic events," but did not directly address Stack's rant against the government agency.

"This incident is of deep concern to me," the statement read. "We are working with law-enforcement agencies to fully investigate the events that led up to this plane crash."

Stack took off in a Piper Cherokee from Georgetown Municipal Airport in Texas at 9:40 a.m. Federal Aviation Administration spokesman Lynn Lunsford said he didn't file a flight plan. The plane crashed into the building in Austin about 20 minutes later.

Click here for chilling eye-witness acccounts of Texas plane crash.

The Department of Homeland Security said it did not believe the crash was an act of terrorism. President Obama was briefed on the incident. As a precaution, the Colorado-based North American Aerospace Defense Command launched two F-16 aircraft from Houston's Ellington Field, and was conducting an air patrol over the crash area.

Patrick Beach, who once played in a band with Stack, described him as a mild-mannered guy who was a stereotypical software guy.

"I talked to alot of people who knew him better than I did, and no one saw anything like this coming," Beach told Fox News.

The toughest part about this, Beach said, was how this guy, who loved his wife and step-child, could be the same person who wanted to "commit mass murder."

Billy Eli, a band member of Stack's, has known the man for about five years and said he never suspected Stack had any political feelings.

"The Joe I knew was mostly apolitical," he told Fox News. "I never heard him talk politics, or take a stand left or right. As far as I know he didn't have a party affiliation."

Stuart Newberg, who was in the area right before the crash, said the plane was flying low and fast when it plowed into the building, according to The Statesman.com.

“It was flying low and fast and I did a double take," Newberg said, according to the Web site.

"I thought it was a play remote control plane. Then I saw the smoke."

He told the paper he thought the plane seemed “very controlled.”

In a neighborhood about six miles from the crash site, a home listed as belonging to Stack was on fire earlier Thursday. Two law enforcement officials said Stack apparently set fire to his home before embarking on his suicide mission.

MyFoxAustin.com said firefighters reported that the entire house was on fire, including the fence, when they arrived on the scene.

Neighbors said they heard a loud explosion in the house Thursday morning right before it became engulfed in flames.

MyFoxAustin.com reported that a 12-year-old girl and a woman were rescued by a neighbor from the $236,000 home. The station reported that the girl is believed to be Stack's stepdaughter. Other media reports indicated that these individuals may have alerted authorities to Stack’s actions.

A neighbor told MyFoxAustin.com that Stack was an experienced pilot who owned his own plane.

The Austin American-Statesman newspaper reported several "walking wounded" at the scene of the crash. Paramedics set up a triage center at the scene.

Early reports that the building housed the FBI field office in Austin turned out not to be true. An FBI spokesman told Fox News that the FBI office in Austin is near where the plane crashed, but not in the same building. There are some federal offices in the building, though authorities couldn't identify which ones.

The NTSB was sending staff out of Dallas and Washington to the scene.

Witnesses were asked to contact the Austin Police Department at 210-650-6196 with any information that might be useful in the investigation.

According to California Secretary of State records, Stack had a troubled business history, twice starting software companies in California that ultimately were suspended by the state's Franchise Tax Board.

In 1985, he incorporated Prowess Engineering Inc. in Corona. It was suspended two years later. He started Software Systems Service Corp. in Lincoln in 1995 and that entity was suspended in 2001. Stack listed himself as chief executive officer of both companies.

Click here for more from MyFoxAustin.com.

NewsCore contributed to this report.

FoxNews.com's Michelle Maskaly, The Statesman.com and The Associated Press contributed to this report.