Smoke billows from a seven-storey building after a private plane crashed into the building in Austin, Texas, on Thursday. (Claudia Grisales/Austin American-Statesman/Associated Press)

Emergency crews have found a second body in the wreckage where a small plane smashed into an Austin, Texas, office building that houses a U.S. Internal Revenue Service office.

Palmer Buck, the Austin Fire Department 's battalion chief, said late Thursday that authorities "have now accounted for everybody," but declined to discuss the identities of those found.

Authorities said earlier that the pilot who crashed into the building was presumed dead and one worker in the building had been missing.

FBI officials identified the pilot as software engineer Joseph Stack, and they are looking at the Austin man's website for a possible motive for Thursday's crash.

They said the site contained a message posted Thursday in which he denounced the IRS and the government, citing earlier problems with the tax department.

"Violence not only is the answer, it is the only answer," said the note on Stack's site.

"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," said the note.

Law enforcement officials said Stack's house, about eight kilometres from the crash site, was set on fire earlier Thursday.

Federal Aviation Administration spokesman Lynn Lunsford said the pilot of the plane did not file a flight plan.

Authorities said 13 people were treated for injuries after Thursday morning's crash. Two victims were in critical condition.

"It felt like a bomb blew off. The ceiling caved in and windows blew in. We got up and ran," said Peggy Walker, a U.S. Internal Revenue Service official who was sitting at her desk in the building when the plane hit.

The building housed about 190 IRS workers, the department said.

With files from The Associated Press