APD: Plane Crash is Criminal Act by One Person

AUSTIN, TX (myFOXaustin) - A small single engine plane crashed into a 7-story office building in North Austin near Mopac and 183. The plane crashed into the Echelon building around 10 a.m. Thursday morning. Officials say 2 people were found dead inside the building and 2 more were transported to hospital.

Moments after the plane hit the building it was fully engulfed. Witnesses told FOX 7 that the plane was flying just above the tree tops before it hit the building.

WATCH: Eyewitness tells FOX 7 plane almost hit his car

The FBI said that they believe Joseph Stack of Austin flew the single engine plane into the building.  Investigators believe Stack was angry at the IRS, which has an office inside the building. Investigators are looking at an anti-government message on the Web linked to him.  The Web site outlines problems with the IRS and says violence "is the only answer."

LINK: Who is Joseph Stack

The Austin Fire Department has recovered two bodies from the Echelon Building Thursday night.  Authorities have not identified the remains.

Austin Police Chief Art Acevedo confirmed Thursday afternoon that one federal worker, an IRS supervisor, was unaccounted for. His family had been notified but his identity was not released. Acevedo said that the prospects of finding that worker were not good.

Authorities refused to say whether or not one of the bodies was that of the missing worker. They also refused to say whether or not the other body was that of the pilot, who is believed to be Stack.

AFD officials said they would remain in the building throughout the night to put out hot spots and watch for fire flare ups.

EMS treated 13 victims at the scene.  Two of those victims had to be taken to the hospital.  One of the victims suffered from smoke inhalation but was expected to be released from the hospital sometime Thursday night.  the other vicitm, identified as Shane Hill, was originally taken to Brackenridge but was later transported to San Antonio as a burn victim.   He had burns over 20 to 25% of his body.  However, the burns were second degree burns and he was expected to recover from them.

The city evaluated the building and has confirmed that it is structurally stable enough to proceed.  Authorities are transitioning the site from a fire scene to a crime scene. 100 officers have been stationed on the perimeter and will remain there until the scene has been settled. 

According to fire officials, the fire spread to multiple floors and crews continue to search 1-3rd floors.  Austin Fire Dept. had instructed Austin Energy to shut down power to the structure as a precaution, but the traffic signals are working again. Agencies are responding to this high-rise fire in normal operation mode and all expect to remain on site for a while.

The FBI is working with numerous other agencies including APD, AFD, Texas Rangers, NTSB, and the FAA among others.  The FBI urges all eye witnesses to call their command post at (512) 974-0699

 "This is an isolated incident, there is no cause for alarm," said Austin Police Chief Art Acevedo.

Federal Aviation Administration spokesman Lynn Lunsford said the pilot didn't file a flight plan. He didn't identify the pilot. As a precaution, the Colorado-based North American Aerospace Defense Command launched two F-16 aircraft from Houston's Ellington Field, and is conducting an air patrol over the crash area.

Police and fire officials evacuated buildings in the surrounding area.