Flames engulf the building after the plane crashed
The pilot of a plane which crashed into an office block in Austin, Texas, left a note expressing his anger at federal tax authorities, police say.
Police are linking the apparent suicide note left online to Joseph Andrew Stack, the man named as the pilot.
The note criticised the Internal Revenue Service - based inside the office block and declared: "Violence is the only answer."
Firefighters continue to search for one person who is still unaccounted for.
The single-engined Piper Cherokee plane hit the second floor of the seven-storey building at 0956 local time (1556GMT).
It had taken off from nearby Georgetown airport in Texas, and did not file a flight plan, Lynn Lunsford of the Federal Aviation Administration said.
Twisted metal and debris seemed to be the only thing left on some floors
Heather Wills, from Austin, told the BBC that she was driving past when she saw the huge cloud of black smoke.
"As I got nearer I could see flames leaping out of the building - the flames were two storeys high. I could hear the glass windows shattering from the heat.
"My first thought was that it was a fire. The traffic was backed up all along the freeway."
Around 190 IRS employees work in the office complex and some were forced to climb out of windows after the plane burst into flames.
Two people were taken to hospital, but it is not clear if they were seriously injured. There has been no official statement on the status of the pilot.
Police are also investigating whether Mr Stack set fire to his house before crashing the plane.
The message on the website apparently registered to and signed by Mr Stack speaks of having problems with the IRS.
"Well, Mr Big Brother IRS man, let's try something different; take my pound of flesh and sleep well," the note reads.
Austin Police Chief Art Acevedo said there was no cause for concern and assured residents that it was an isolated incident.
The White House said the crash did not appear to be an act of terrorism.
White House spokesman Robert Gibbs said President Barack Obama had been briefed about the incident.
The Department of Homeland Security was investigating the crash, he added.
As a precaution, the North American Aerospace Defense Command scrambled two F-16 fighter jets from Houston, Texas, to patrol the area.