Eighteen months ago, John Dullahan was an intelligence analyst with a long and varied career in both the military and the classified world. Today, he is jobless and blacklisted from the federal workforce, his loyalty to the United States, he says, brought into question.

He just isn't sure why.

On St. Patrick's Day 2009, the government stripped the Irish-born Dullahan's security clearance and fired him from his job at the Defense Intelligence Agency in a manner that has no precedent at the Pentagon - invoking a national security clause that states that it would harm the interests of the United States to inform him of the accusations against him.

As a result, Dullahan, a Vietnam veteran who served at military posts around the world and as a U.N. weapons inspector in Saddam Hussein's Iraq, cannot appeal to a board of senior agency officials, as others in his position might. He is, in effect, stranded.

"This has been devastating for me," said Dullahan, 65, who became a U.S. citizen in 1973. "I am a loyal American."

Security clearances are a ticket to opportunity for hundreds of thousands of federal workers and contractors. But when those clearances are taken away, so is any chance of employment in the national security complex.

The reasons to revoke a security clearance can vary. Some federal workers lose them because they are found to be using illicit drugs. Others lose them when they are determined to be financially vulnerable, a situation that might make them susceptible to blackmail.

Dullahan was fired after apparently "showing deception" during three polygraphs - each time when he was asked whether he had ever spied for the Soviet Union. That, at least, is his best guess as to the reason for his termination.

"We just don't know what it was that caused DIA to do what it did," said Mark Zaid, Dullahan's attorney. "Presumably it's connected to the polygraph, but at what level we just don't know. What happened to John is extremely rare. It doesn't happen. I've had dozens, if not hundreds, of cases where people have failed their polygraph and been accused of drugs, espionage, you name it, and they have all gotten due process."

The Defense Department declined to comment on Dullahan's case but acknowledged that the procedure used against him is rare, if not unprecedented.

"As intended by Congress, this statute is only to be used in a manner consistent with the national security," the Defense Department said in a statement. "There is only one known invocation of employing this statute as a basis for termination."

But the department would not confirm that the one case in the 14 years of the statute's existence is, in fact, Dullahan's, citing privacy reasons. Typically, employees who lose their clearances and are terminated are given some explanation, which they often use to challenge the decision.