If the contractors were the aggressors in an incident, they could be deemed to be unlawfully using offensive force, said Scott Silliman, a retired Air Force lawyer and now a professor at Duke University. He said they could claim self-defense only if they had been fired on.
"The only force they can use is defensive force," Silliman said. "But we may be seeing some instances where contractors are using offensive force, which in my judgment would be unlawful."
Some current Defense Department lawyers think that interpretation is too restrictive. Like soldiers, the security guards should be able to defend themselves if they detect "hostile intent," said the Defense official.
But the guards often participate in operations where the line between defensive and offensive force is hard to determine, such as escorting a diplomat through a neighborhood in a war zone, as many frequently do.
Those operations may be best left to the military, some experts said.
"To what extent is it appropriate to have people not in the chain of command under the president of the United States involved in the application of force?" the official asked.
Any doubt on the legal status of the contractors is likely to open the United States to further criticism from the international community.
John Hutson, a former top Navy lawyer, said he did not consider contractors to be unlawful combatants.
But that will be a difficult argument for U.S. officials to make, he emphasized.
"We are going to be hard-pressed to draw a distinction between the guys in Blackwater carrying automatic weapons and the bad guys setting bombs along the side of the road," said Hutson, now dean of Franklin Pierce Law Center in New Hampshire.
U.S. officials have described many of the suspected Al Qaeda and Taliban affiliates it holds at Guantanamo Bay as unlawful combatants either for taking part in hostilities against the United States or by supporting the hostilities while not part of a nation's military.
By that standard, some of the private guards in Iraq and Afghanistan also could be seen as unlawful combatants, particularly if they have taken offensive action against unarmed civilians, experts said.
"If we hire people and direct them to perform activities that are direct participation in hostilities, then at least by the Guantanamo standard, that is a war crime," Schmitt said.
The 2004 immunity measure prevents Iraq from prosecuting private guards under Iraqi law. But some international law experts think Iraq could use international treaties to try contractors for killing civilians.
For now, such trials are considered unlikely, especially because the Iraqi government does not have the contractors in custody.
Many of the current and former federal officials think the administration has an obligation under the Geneva Convention to clarify the contractors' status. Some are perplexed that the Bush administration did not resolve these issues -- or at least discuss them more thoroughly -- before putting contractors on such a complex battlefield.
