The building housing nearly 190 federal tax employees was engulfed with fire and smoke sending the employees scurrying for safety.
The software engineer and the pilot of the plane identified as Joseph Stack, is believed to have been killed in the crash. Besides this, one person who worked in the building was unaccounted while two people were hospitalized.
Investigators are looking into a anti-government message that Stack posted on the Web, speaking on his problems with the IRS.
"Violence not only is the answer, it is the only answer," the long note on Stack's Web site reads.
The note dated Thursday, Feb 18, goes on,"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."