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By Darrell Preston and Justin Blum Feb. 18 (Bloomberg) — Federal authorities are investigating a Web site suicide note and an Austin, Texas, house fire linked to a pilot who crashed his small plane into a building in the Texas capital containing offices of the U.S. Internal Revenue Service, said a law enforcement official with knowledge of the investigation. The pilot was identified as Joseph A. Stack, said the law- enforcement official, who spoke on the condition of anonymity because he was not authorized to comment publicly. Investigators are probing whether Stack set fire to his house before crashing the plane, the official said. Spokesmen for the Federal Bureau of Investigation and the Department of Homeland Security said that initial indications show no ties to organized terrorism. “It does not appear to be terrorism,” White House spokesman Robert Gibbs told reporters traveling with President Barack Obama on Air Force One today. The Department of Homeland Security “is looking at all angles,” he said. Authorities are looking into a Web site suicide note signed “Joe Stack (1956-2010)” that may have been posted by the 53- year-old Stack, which described his anguish over years of battling with the Internal Revenue Service. “Violence not only is the answer, it is the only answer,” the letter said. “Well, Mr. Big Brother IRS man, let’s try something different; take my pound of flesh and sleep well.” One Unaccounted For Austin police said in a news conference that they have accounted for all but one person believed to be in the building when it was hit by the plane. They were trying to determine whether that person had been at work before the accident. The IRS said in a statement it employed 190 people in the Echelon 1 building in Austin. North American Aerospace Defense Command, which oversees defense of airspace over the United States and Canada, said in a statement that it launched two F-16 fighter aircraft from Ellington Field in Houston in response to the crash. Witnesses interviewed by the Associated Press described windows exploding and flames shooting out after the plane rammed the building. “It felt like a bomb blew off,” Peggy Walker, an IRS revenue officer, told the AP. Walker said she was sitting at her desk in the building when the plane hit. “The ceiling caved in and windows blew in. We got up and ran,” she said. In witness accounts posted on the Austin American-Statesman newspaper Web site, William Winnie, identified as an Internal Revenue Service agent, described watching the plane come straight at the building from a window on the third floor. “It looked like it was coming right in my window,” Winnie told the Statesman. The plane turned and dove into the floors below him, Winnie said. ‘Isolated Incident’ The plane was a Piper Cherokee PA 28 that took off from the airport in Georgetown, 30 miles north of Austin, at 9:40 a.m., said Diane Spitaliere, a Federal Aviation Administration spokeswoman, in an interview in Washington. The plane wasn’t stolen as initially reported by local news outlets, said Art Acevedo, Austin police chief, in a press conference broadcast on the Internet. “I can tell you categorically there is no concern from a law enforcement or terrorism perspective,” said Acevedo. “This is an isolated incident.” Firefighters were called to a burning home in Austin before the crash. Public records showed the house was owned by Andrew Joseph Stack. The American-Statesman reported that a neighbor ran to the house when he saw it in flames, and later saw a woman and young girl drive up in a car. Neighbors believed the two to be Stack’s wife and daughter, the paper reported. A Red Cross spokeswoman said they were assisting two people who were “remarkably calm, clearly distraught” and “physically fine,” the paper said. To contact the reporters on this story: Justin Blum in Washington at jblum4@bloomberg.net ; Darrell Preston in Dallas at dpreston@bloomberg.net ;