Cleland, 28, used to escort prisoners in Guantanamo Bay, Cuba, as a platoon sergeant in the Marine Corps. But in May he was behind bars in Fallujah, Iraq, his Marine Corps brothers became his guards, and Cleland suddenly identified with his former captives.

''I'm sure those guys at Gitmo were thinking the same thing: How long am I going to be here?" said Cleland, of Salem, Ohio, who had left the Corps in 2002 and joined Zapata Engineering, one of many security contractors working in Iraq.

Cleland and 15 other US contractors were taken into custody by a Marine unit May 28, when military officials alleged that they fired at a Marine checkpoint as their convoy passed through Fallujah -- the first public accusation of that kind. It gave Cleland and his compatriots a rare glimpse of life as detainees, and the humiliation, fear, and despair that come with it.

The contractors also became caught in a simmering power struggle between active-duty military personnel and the more than 20,000 well-paid private security operatives who work in Iraq. The contractors operate outside the military chain of command and are not subject to military law, which can lead to resentment from US forces and confusion in the field. Contractors, many of them veterans of years in combat, contend that young US troops lack their experience and judgment under pressure. Yet each group cannot carry out its mission in a hostile Iraq without the other.

The tension may have spiked on that evening six weeks ago, when the Marines allege that the 16 contractors, all of whom had military experience, fired wildly on civilians and US observation posts. The contractors contend they were returning to their base and would never fire at US troops.

The Marine Corps is investigating. The Zapata contractors were released but blacklisted, banished from the security field in Iraq and branded as criminals, though none has been charged with a crime.

This account is based on interviews with eight of them.

The convoy of four Ford F-350 pickup trucks and one armored Ford Excursion left Zapata's base outside Fallujah on May 28 with a fairly standard mission: Drop off a small amount of explosives at the massive US base named Camp Victory near Baghdad International Airport and pick up a few Iraqi civilian employees. Then turn around and come back.

The first leg of their trip was fairly uneventful. Convoy members say they had arrived at Camp Victory and were grabbing lunch by 2 p.m. It was around that time, the Marines would later allege, that a convoy matching the description of the Zapata vehicles began indiscriminately firing at civilians. The contractors say that it never happened and that they had no hint of trouble until the drive back.

''Before we left [Camp Victory], I called my wife and said: 'I should be there in about half an hour. I'll call you then,' " said Pete Ginter, an eight-year Marine veteran who had worked as a contractor in Bosnia and Kosovo before moving to Iraq. ''That was the last time I talked to her for 3 1/2 days."

Problems began for the contractors when, on the return trip through Fallujah later that afternoon, they spotted a front-end loader approaching from the right side. Fearing an ambush, Cleland, armed with an M-4, leaned out his window and tried to wave off the truck. When he could not get the driver's attention, he fired three shots into the ground.

The Marines tell a different story, saying the observation post ''was fired on by gunmen from vehicles matching the description of those involved in the earlier attack."

According to Marine statements released after the incident, Marines also observed the convoy shooting at civilian vehicles in Fallujah.

Members of the convoy said they believe they received fire as they crossed the bridge, but did not return it. Once they arrived on the west side of the bridge, the contractors snaked through the barricades leading to the checkpoint, and the fourth vehicle, a Ford pickup, grazed a strip of spikes designed to slow vehicles as they approach. One of the pickup's rear tires exploded.

But there was no alarm from the Marines manning the post, according to the contractors. The convoy members changed their blown tire and spoke with a young Marine who asked if they needed help. Several minutes later, a Marine captain came out, accused the men of shooting at his post, and ordered them to a base in Fallujah.

Still armed, the contractors were taken to the base, some riding in a Humvee with a hooded and restrained insurgent suspect. There, they were held in a conference room and handed over their official Defense Department identification cards. Their weapons were stashed elsewhere.

One by one, the contractors were escorted into a courtyard where spotlights shone against a dark sky. Several contractors said a group of perhaps 40 Marines was standing in a half-circle, jeering and heckling. The contractors said they were thrown to their knees, and their hands were tethered with plastic handcuffs. .

Marine Lieutenant Colonel Dave Lapan, a spokesman for Multi-National Force-West, said Major General Stephen T. Johnson, his unit's commanding general, ordered the security contractors held pending an investigation.

''Their actions were deemed a threat to Marines and others," Lapan said. ''We are investigating previous incidents of a similar nature."