When is crashing a plane into a building in an act of explicit political violence not terrorism?

There must be some fundamental difference between this guy and the 9/11 terrorists (besides sheer scale), but somehow I can’t quite put my finger on it.

(CNN) — An Austin, Texas, resident with an apparent grudge against the Internal Revenue Service set his house on fire Thursday and then crashed a small plane into a building housing an IRS office with nearly 200 employees, officials said.

Federal authorities identified the pilot of the Piper Cherokee PA-28 as Joseph Andrew Stack, 53.

Two people were injured and one person was missing, local officials said. There were no reported deaths.

A message on a Web site registered to Stack appears to be a suicide note.

See iReport photos and videos from the scene

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

In the lengthy, rambling message, the writer rails against the government and, particularly, the IRS.

See text of the note (PDF)

The building into which the airplane crashed is a federal IRS center with 199 employees.

Two F-16 fighter jets were sent from Houston as a precaution, but federal authorities said preliminary information did not indicate any terrorist connection to the crash.

“We do not yet know the cause of the plane crash,” the Department of Homeland Security said in a release. “At this time, we have no reason to believe there is a nexus to terrorist activity. We continue to gather more information, and are aware there is additional information about the pilot’s history.”