The plane appears to have hit the first-floor offices of the Internal Revenue Service. Eye-witnesses say the plane, which took off from a nearby airport, was coming in at full throttle — though the FBI is still investigating it as an accident.

It is still unclear how many have been injured in the strike.

Major Garrett Tweets that NORAD has scrambled 2 F-16 fighter jets from Ellington Field in Houston, which they called “a prudent precaution and consistent with our response to recent similar air incidents.”

UPDATE: Federal officials are now investigating the strike as a criminal act:

An Federal Aviation Administration spokesman said the crash was “apparently a criminal act” and that in such a case the FBI would be the lead investigating agency rather than the National Transportation Safety Administration.
“The NTSB investigates accidents,” said Paul Turk, assistant director of public affairs for the FAA.
Turk could not confirm that the plane that hit the building belonged to Joseph Andrew Stack, the private plane pilot whose nearby home was on fire at roughly the same time.
“We don’t have a tail number for the aircraft involved,” Turk said.
The pilot of the plane that crashed did not file a flight plan or, as far as FAA officials know at this time, have any other contact with the agency.

The pilot has been identified as Joseph Andrew Stack, a software engineer who apparently set his house ablaze — with his daughter inside — before setting off on the flight.

UPDATE II: The Austin Statesman is pointing to an angry political rant posted by Stack, the alleged pilot of the plane. The note takes shots at George W. Bush, Enron, GM, the health-care system, the Justice Department, the late Senator Patrick Moynihan, the Catholic church and, most stridently, the IRS.

UPDATE III: CBS’s Mark Knoller tweets that the president has been briefed on the Austin incident and that the White House does not consider it an act of terrorism.