Furious with the US tax agency Internal Revenue Service (IRS), a software engineer crashed his small plane into a seven-storey building in Texas housing nearly 200 federal tax employees, igniting a raging fire that sent workers running for their lives.
The suicide attack left two people injured inside the building and another person unaccounted for.
Officials in Houston are investigating whether the pilot, identified as Joseph Andrew Stack, 53, crashed the plane intentionally. Stack was confirmed dead.
At least one person who worked in the building was unaccounted for and two people were hospitalised, according to Austin Fire Department Division Chief Dawn Clopton.
A US law official said investigators were looking at a lengthy, anti-government "manifesto" Stack is believed to have written on his Website. The message outlines problems with the IRS and says violence "is the only answer".
The site is titled, "Well Mr. Big Brother IRS man ... take my pound of flesh and sleep well."
The note and other documents indicate Stack moved from California to Austin after the dot-com bust. Calls to the accountant and Stack's ex-wife were not immediately returned. Austin police declined to discuss the house fire and Internet manifesto.