I think it was not a terroristic type of attack. He did not attack the IRS building to cause fear as many who use terroristic type of attacks do. This was just a guy on a suicide mission who wanted his moment of fame. He was not looking to cause mass chaos or people running away in fear. His was a simple mission and that was to cause the IRS and the federal government to feel some pain that he has been feeling for awhile.
Terrorist type attacks are a much different style of attack. I know there are some similarities but you have to look deeper into the cause and reason for the attack.