U.S. Marines and employees of a contractor working for the military in Iraq offer sharply contrasting versions of a recent incident in which Marines in Fallujah detained 19 of the company's employees for three days after saying the contractors fired at them.

The contractors say they did not, and some now allege that the Marines abused them during their confinement.

There have been reports before of tension and miscommunication between U.S. forces and the thousands of armed contractors in Iraq, but this is believed to be the first publicly acknowledged case of U.S. contractors being detained for allegedly firing at U.S. soldiers.

No charges have been filed, but the Pentagon and the contracting firm, Zapata Engineering of Charlotte, N.C., are investigating, and at least two of the contractors have hired a lawyer.

According to the Marines' account, Marines in Fallujah came under small-arms fire from gunmen in several trucks and sport-utility vehicles on May 28. The gunmen also fired at civilian cars. Later that afternoon, the vehicles came back and gunmen fired on Marines at a different observation post. No one was hit, but the vehicles were stopped. Sixteen Americans and three Iraqis -- all Zapata employees -- were taken into custody, and held at the detention facility Camp Fallujah until their release on May 31.

"In accordance with standard operating procedures, the Americans were segregated from the rest of the detainee population and, like all security detainees, were treated humanely and respectfully," Lt. Col. David A. Lapan, a Marine spokesman, said in a written statement.

The contractors disputed that account. They said they never fired at Marines and were mistreated while in custody.

Mark Schopper, a lawyer who represents two of the detainees, said Marines threw contractors roughly to the ground, jammed knees into their backs, taunted them and denied requests to call their families. Schopper, whose clients are both former Marines, said one of his clients "had his testicles squeezed so hard that he nearly passed out from the pain."

At one point during the contractors' confinement, Schopper said, a Marine asked, "How does it feel to be a rich contractor now?"

"These guys are making twice as much as their former commanders. That's bound to bring animosity," Schopper said.

Zapata Engineering President Manuel L. Zapata said in a written statement that the firm will investigate both the alleged shooting and the alleged abuse, but "we cannot imagine that our employees would fire on American service members." Of the 19 detained employees, 14 were armed security workers and eight were former Marines.

Zapata said that he thinks "the root cause of the events was a misunderstanding by people who are living and working in an intense and stressful situation."

The company, which has a contract with the U.S. Army Corps of Engineers to manage an ammunition storage depot in Iraq, said none of its employees was injured. All have left Iraq and will no longer work for the company. A company spokeswoman said they were not fired.

The more than 20,000 private military contractors in Iraq have played a critical role in the two-year-long U.S. occupation, doing such things as guarding high-level officials, interrogating prisoners and running supply lines. At times, relations between soldiers and contractors have been strained. Contractors work outside the military's chain of command, and generally earn much higher pay than soldiers.

Peter W. Singer, a Brookings Institution fellow who has written a book about military contractors, said the incident highlights legal ambiguities about contractors' roles that still haven't been resolved. If the contractors did fire on Marines, Singer said, there's no obvious way to deal with it because neither military law nor local Iraqi law applies to contractors. There is a U.S. law that might apply, but it's never been tested against contractors. "The Marines don't have a legal recourse over there. They can't just court-martial them," he said.

The Pentagon issued regulations last month intended to better define the limits of contractor behavior in the field. Sen. Patrick J. Leahy (D-Vt.) said yesterday that the government needs to further clarify the rules.

"There have been incidents when U.S. military personnel have mistakenly been shot at by contractors, and vice versa," Leahy said in a written statement. "Accountability is too much an afterthought in these arrangements."

The Naval Criminal Investigative Service has begun an investigation into the initial incident and the contractors' allegations, according to Lt. Cmdr. Danny Hernandez, a Navy spokesman.

Staff researcher Meg Smith contributed to this report.