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A "cowardly, criminal" act ... or domestic terrorism?

That's one of many questions being asked in the wake of Thursday's plane crash in Austin, Texas, where a software engineer with an anti-government grudge crashed his single-engine plane into an office building that housed nearly 200 Internal Revenue Service employees.

The pilot, 53-year-old Joseph Stack, killed both himself and a 67-year-old IRS employee, Vernon Hunter, in the attack.

Stack is believed to have posted an online manifesto just hours earlier, detailing his longstanding gripes with "big brother," the "IRS man" and the Catholic church.

White House Press Secretary Robert Gibbs told reporters aboard Air Force One that White House officials will let an investigation "play out" before determining how to label the incident. He had earlier said that the incident did not appear to be terrorism, and when asked specifically if domestic terrorism was a possibility, he said he did not suspect ''somebody like an Al Qaeda.''

Austin Police Chief Art Acevedo, meanwhile, labeled the incident a single act by a lone individual and refused to classify it as terrorism.

"I call it a cowardly, criminal act, and there was no excuse for it," he told reporters.

SLIDESHOW: Small Plane Crashes Into Austin Office Building

But at least one lawmaker, U.S. Rep. Lloyd Doggett, D-Texas, disagreed and compared the incident to the 1995 bombing that killed 168 people in the Alfred P. Murrah Federal Building in Oklahoma City.

"Like the larger-scale tragedy in Oklahoma City, this was a cowardly act of domestic terrorism," Doggett said in a statement. "Stack's apparent website message reflects the steadily increasing flow of 'the government is out to get me' paranoia."

Matthew Chandler, a spokesman for the Department of Homeland Security, referred FoxNews.com to Gibbs' comment when asked to clarify why the incident has not been labeled an act of domestic terrorism.

Ken Gude, a senior fellow at the Center for American Progress, a liberal think tank based in Washington, said that while it's appropriate for Gibbs to reserve judgment on the matter, Stack's act of violence has all the markings of domestic terrorism.

"It certainly appears as if his motives would qualify as domestic terrorism," said Gude, a former policy analyst at the Center for National Security Studies. "He put out an anti-IRS creed, he used an airplane to crash into a building with the clear intention of causing destruction and death."

But Gude urged caution before placing any label on the event, which he characterized as an inexcusable, unacceptable act that "should not be tolerated" in any way whatsoever. He said the Austin crash differed from the Oklahoma City bombing because Stack apparently acted alone. (Militia sympathizer Timothy McVeigh was executed in 2001 for organizing the Oklahoma City bombing; his accomplice, Terry Nichols, was sentenced three years later to 161 consecutive life terms without the possibility of parole for assisting him.)

DHS officials explicitly warned against the potential emergence of terrorist groups or "lone wolf extremists" in a report issued in April. The report, "Rightwing Extremism: Current Economic and Political Climate Fueling Resurgence in Radicalization and Recruitment," found that while no specific threat existed, there was the potential for violence from extremists concerned about illegal immigration, abortion, increased federal power and restrictions on firearms.

The report followed a similar DHS document released in January 2009 that detailed left-wing threats, focusing on cyberattacks and radical "eco-terrorist" groups like Earth Liberation Front.

Jena Baker McNeill, a homeland security policy analyst for the Heritage Foundation, a conservative think tank based in Washington, said it's "too early" to make a definitive determination of whether Thursday's incident was a criminal or terrorist act.

"I would also await more details from the investigation," McNeill wrote in an e-mail. "I will say that the number of fatalities should not have an impact on whether something is labeled an act of terrorism — but again, still waiting on more details before a conclusion can be made."

Gude echoed that sentiment, saying the focus shouldn't be on casualty rates, but rather on the source of the extremism.

"There has always been an undercurrent of anti-government groups in the United States," he said. "It's appropriate for our law enforcement, intelligence and security agencies to be vigilant and be aware that this threat is real."

Gude continued, "There is concern, certainly, about the volume of extremist views that seems to be percolating in the political space in the United States. But it's also evident that this type of threat from extremism has existed for a very long time and it will be with us for a very long time."

Several news reports likened Thursday's event to that of Charles Bishop, the teenage Al Qaeda sympathizer who flew a small plane into a Bank of America building in Tampa, Fla., in 2002, and Johnny Lee Wicks, a 66-year-old shotgun-wielding retiree who killed a security guard and wounded a deputy U.S. marshal at a Las Vegas courthouse in January. FBI officials have said Wicks was angry about losing a government lawsuit in which he had challenged a cut in his monthly Social Security benefits.