The first court-martial in the Iraqi prison abuse scandal is set to open Wednesday. A U.S. Army soldier will be tried by a military court in Baghdad.

According to the U.S. Army's investigation outlining abuse of the detainees at Baghdad's Abu Ghraib prison, some soldiers working at the jail say civilian contract interrogators and translators encouraged the abuse of prisoners.

John Hutson, a former U.S. Navy lawyer and now dean of the Franklin Pierce Law Center, says that if contractors did help encourage prisoner abuse, their legal status for any criminal prosecution is murky.

"It's something that frankly we did not anticipate very well, and I think we have left ourselves very vulnerable," he said. "And it comes at a particularly bad time because we are using contractors more and more and more and more to perform what are essential, fundamental military functions, so that this issue is likely to come up frequently, unfortunately."

There are no official figures available, but experts believe there to be at least 20,000 contractors working in security-related jobs in Iraq. Those who are not hired locally, such as facilities guards, are primarily retired military or intelligence personnel.

Deborah Avant, a political science professor at George Washington University and a specialist on military privatization, says the U.S. military's use of security contractors exploded in the 1990s as the overall U.S. troop strength was reduced. She says it picked up steam since September 11, 2001, particularly with the demands and dangers of the Iraq occupation.

"Partly this is because the U.S. government is doing more and is hiring more contractors to go along with it," said Ms. Avant. "But it's also the fact that many more different kinds of entities, private sector, NGOs [non-governmental organizations], national organizations, are hiring their own security. And so these contractors work not only for governments."

But legal authority over civilian defense contractors remains unclear. They are exempt from Iraqi courts. They are not subject to the military justice system except in wartime, and a military appeals court has ruled it has to be a war declared by Congress, something that hasn't happened since 1941. Mr. Hutson says that leaves an awfully big legal loophole."

"The president has said there's a war, Congress has given us money for the war, so that it's arguable," he said. "But it's far, far, far from being perfect. There's a huge hole, a jurisdictional hole, that you could drive a tank through, potentially."

Congress attempted to partially close that loophole with a new law passed in 2000 that allows defense contractors working for U.S. forces to be prosecuted in federal courts. The Bush administration contends the new law is sufficient to prosecute any crimes committed by civilian contractors in Iraq. But, as Mr. Hutson points out, the law is untested, and it may not cover some of the offenses, such as assault, allegedly perpetrated at Abu Ghraib.

"The problem with that is that it's never been used," he said. "It's never been implemented by DOD [Department of Defense], there have been no implementing directives since Congress passed it. So that is a little less than perfect as a remedy."

In addition, that law applies only to contractors working for the Defense Department, not to those working for other non-Defense agencies such as the CIA.