A man angry with the Internal Revenue Service flew a small airplane into a building on Thursday morning in Austin, Texas, in which several IRS offices are located, killing an IRS employee and himself.

The IRS has offices on the first floor of the building, including its civil enforcement and criminal investigations divisions, according to the Austin Statesman. Most employees were accounted for, but one IRS worker was killed, according to the Associated Press. Firefighters searching the building located the remains of the pilot and the IRS employee. Thirteen people were injured, two of them critically as of Thursday night.

"We can confirm that a small plane hit a building in Austin, Texas that includes IRS offices," said IRS spokesman Bruce I. Friedland on Thursday afternoon. "This is the Echelon 1 Building, which houses about 190 IRS employees. We are still in the process of accounting for all of our employees. We will be providing updates as more information becomes available."

Law enforcement officials initially said the crash did not appear to be an act of terrorism, but later discovered that the pilot of the plane had set his own house on fire before takeoff. His wife and daughter were seen outside the house and appeared distraught, according to a neighbor.

The suspect, Andrew Joseph Stack III, 53, had also posted an online diatribe against the IRS, shortly before setting fire to his home, writing, "I know I am hardly the first one to decide I have had all that I can stand." In the document, signed  "Joe Stack (1956-2010)," he complained about Section 176 of the Tax Code on the treatment of technical personnel, and how it affected his taxes as a contract software engineer in the 1980s. He said that he had joined a campaign against the tax provision in 1987, and "spent close to $5,000 of my 'pocket change' and at least 1,000 hours of my time writing, printing and mailing to any senator, congressman, governor or slug that might listen; none did, and they universally treated me as if I was wasting their time."

The document was titled, "Well Mr. Big Brother IRS man ... take my pound of flesh and sleep well." In the document, he blamed the IRS for causing him to lose tens of thousands of dollars in retirement savings over three decades. Stack had started two software businesses in California that were suspended by state tax authorities for nonpayment of back taxes and failure to file a tax return.

An eyewitness reported that Stack's Piper Cherokee plane appeared to take a turn and head in a controlled manner directly for the building. “It wasn’t heading into the direction of the building, but all of a sudden it took a right and headed straight into it,” Susan Whelan told the Austin Statesman. “It didn’t look like it was in distress. It wasn’t wavering at all.”

“It felt like a bomb blew off,” IRS revenue officer Peggy Walker told the AP. “The ceiling caved in and windows blew in. We got up and ran.”

The blaze took over 75 minutes for firefighters to get under control, and they were still putting out hot spots as of Thursday night.

IRS Commissioner Douglas Shulman expressed his shock at the attack. "Like most Americans, I am shocked by the tragic events that took place in Austin this morning," he said in a statement. "This incident is of deep concern to me. We are working with law-enforcement agencies to fully investigate the events that led up to this plane crash. My thoughts and prayers go out to the dedicated employees of the IRS who work in the Austin building. We will immediately begin doing whatever we can to help them during this difficult time. While this appears to be an isolated incident, the safety of our employees is my highest priority. We will continue to do whatever is needed to ensure our employees are safe."