AUSTIN, Texas — A low-flying small plane today crashed into an office building that houses the Internal Revenue Service in Texas, and officials said they were investigating whether it was an intentional act by the pilot.

The U.S. law enforcement officials said authorities were trying to determine if the pilot intentionally targeted the IRS. They spoke on condition of anonymity because the investigation is continuing.

Assistant Austin Fire Chief Harry Evans said at least one person was missing and two people were taken to a hospital. Their conditions and identities were not immediately known.

The crash sent workers fleeing as ceilings crumbled, windows shattered and flames shot out of the building.

Thick black and gray smoke was billowing out of the second and third stories of the building as fire crews using ladder trucks and hoses battled the blaze. Dozens of windows were blown out of the hulking black building.

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.

Peggy Walker, an IRS revenue officer who works in the building said she was sitting at her desk when the plane crashed.

“It felt like a bomb blew off. The ceiling caved in and windows blew in. We got up and ran,” she said.

Matt Farney, 39, who was in the parking lot of a nearby Home Depot, said he saw a low-flying small plane near some apartments and the office building just before it crashed.

“I figured he was going to buzz the apartments or he was showing off,” Farney said. “It was insane. ... It didn’t look like he was out of control or anything.”

Sitting at her desk in another building about a half-mile from the crash, Michelle Santibanez said she felt vibrations after the crash. She and her co-workers ran to the windows, where they saw a scene that reminded them of the 2001 terrorist attacks, she said.

“It was the same kind of scenario with window panels falling out and desks falling out and paperwork flying,” said Santibanez, an accountant.

Fire crews were inside the building, which is located next door to a building that houses the FBI, and looking for survivors, Evans said.

National Transportation Safety Board spokesman Peter Knudson said an investigator from the board’s Dallas office has been dispatched to the scene of the accident to start an investigation.

The IRS Web site said an office of its EP Team Audit Program is located in the building where the plane crashed. The group, known as EPTA, examines employee benefit plans with 2,500 or more participants, according to the Web site.