Texas suicide plane attack on tax 'big brother'

• Firefighters attend to the blaze after a plane crashed into an IRS building in Austin, Texas

The plane hit the building in Austin, Texas, just before 10am local time, setting it ablaze and causing windows to explode.

Workers scrambled to safety after the collision as thick smoke billowed out of the second and third storeys and fire crews battled the blaze.

"It felt like a bomb blew off," said Peggy Walker, an IRS revenue officer, who was sitting at her desk in the building when the plane crashed. "The ceiling caved in and windows blew in. We got up and ran."

Witnesses said that though the plane was close to the building it was flying in a controlled manner, as if the pilot was "showing off".

Gerry Cullen, 66, was in a restaurant across the street when the plane hit. He said: "The airplane hit and vanished in a fireball."

Sitting at her desk in another building, about half a mile from the crash, Michelle Santibanez said she felt vibrations after the crash. She said the scene reminded her of the 11 September 2001 terrorist attacks. She said: "It was the same kind of scenario, with window panels and desks falling out, and paperwork flying."

• Final message of man with a deadly grievance

The White House, however, said the crash did not appear to be an act of terrorism.

Despite the impact, almost all of the office's 190 employees were evacuated safely, though one person remained unaccounted for last night and two people were taken to hospital.

The pilot, identified as Joseph Stack, is thought to have died in the crash. Federal Aviation Administration spokesman Lynn Lunsford initially said the plane was so badly destroyed as to be almost unidentifiable.

Federal law enforcement officials were now investigating whether the pilot crashed on purpose in an effort to blow up the IRS offices.

Mr Stack apparently left a lengthy suicide note on a website, outlining his reasons for the act.

Opening with the sentence: "If you're reading this, you're no doubt asking yourself, 'Why did this have to happen?'" the rambling letter – as yet unauthenticated – sets out Mr Stack's frustration with the government, tax system and politicians, whom he blamed for difficulties his software engineering business had faced.

Explaining why he had decided to kill himself in such a manner, Mr Stack wrote: "I choose to not keep looking over my shoulder at 'big brother' while he strips my carcass, I choose not to ignore what is going on all around me, I choose not to pretend that business as usual won't continue; I have just had enough."

Continuing, he said: "Sadly, violence not only is the answer, it is the only answer." He concluded the letter: "Well, Mr Big Brother IRS man, let's try something different; take my pound of flesh and sleep well," signing it: Joe Stack (1956-2010).

White House spokesman Robert Gibbs said president Barack Obama had been briefed about the incident and the department of homeland security was investigating all angles of the crash and its cause.

Meanwhile, two F-16 fighter jets were scrambled after the crash and were patrolling the area.

Police officials said they were checking reports that Mr Stack was involved in a domestic dispute in Georgetown, Texas, and that before taking off in the plane, had burned down his home.