Terror attack in Texas?

According to a New York Times report, small airplane piloted by Joseph A. Stack III., 53, dove into an office building in Austin, TX. The building housed a local office of the Internal Revenue Service, an agency with which Mr. Stack had a conflict. Stack died because of the crash.

The incident was not judged to be a terrorist act, according the Times report. I wonder why?

The video below originated at CNN.com: