WASHINGTON -- The helicopter was hovering over a Baghdad checkpoint to the Green Zone, one typically crowded with cars, Iraqi civilians and U.S. military personnel.
Suddenly, on that May 2005 day, the copter dropped CS gas, a riot-control substance the U.S. military in Iraq can use only under the strictest conditions and with the approval of top military commanders. An armored vehicle on the ground also released the gas, temporarily blinding drivers, passers-by and at least 10 U.S. soldiers operating the checkpoint.
"This was decidedly uncool and very, very dangerous," Capt. Kincy Clark of the Army, the senior officer at the scene, wrote later that day. "It's not a good thing to cause soldiers who are standing guard against car bombs, snipers and suicide bombers to cover their faces, choke, cough and otherwise degrade our awareness."
Both the helicopter and the vehicle involved in the incident at the Assassins' Gate checkpoint were not from the U.S. military, but were part of a convoy operated by Blackwater Worldwide, the private security contractor that is under scrutiny for its role in a number of violent episodes in Iraq, including a September shooting in downtown Baghdad that left 17 Iraqis dead.
None of the U.S. soldiers exposed to the chemical, which is similar to tear gas, required medical attention, and it is not clear whether any Iraqis did.
Still, the previously undisclosed incident has raised new questions about the role of private security contractors in Iraq and whether they operate under the same rules of engagement and international treaty obligations that the U.S. military observes.
"You run into this issue time and again with Blackwater, where the rules that apply to the U.S. military don't seem to apply to Blackwater," said Scott L. Silliman, the executive director of the Center on Law, Ethics and National Security at the Duke University School of Law.
Officers and noncommissioned officers from the 3rd Infantry Division who were involved in the episode said there had been no signs of violence at the checkpoint. Instead, they said, the Blackwater convoy appeared to be stuck in traffic and might have been trying to use the riot-control agent as a way to clear a path.
Anne Tyrrell, a spokeswoman for Blackwater, said the CS gas had been released by mistake.
"Blackwater teams in the air and on the ground were preparing a secure route near a checkpoint to provide passage for a motorcade," Tyrrell said in an e-mail message. "It seems a CS gas canister was mistaken for a smoke canister and released near an intersection and checkpoint."
She said the episode was reported to the U.S. Embassy in Baghdad and the embassy's chief security officer and the Defense Department conducted a full investigation. The troops exposed to the gas also said they reported it to their superiors.
But military officials in Washington and Baghdad said they could not confirm that an investigation had been conducted. Officials at the State Department, which contracted with Blackwater to provide diplomatic security, also could not confirm that an investigation had taken place.
Blackwater says it was permitted to carry CS gas under its contract at the time with the State Department. According to a State Department official, the contract did not specifically authorize Blackwater personnel to carry or use CS, but it did not prohibit it.
The military, however, tightly controls use of riot-control agents in war zones. They are banned by an international convention on chemical weapons endorsed by the U.S., although a 1975 presidential order allows their use by the U.S. military in war zones under limited defensive circumstances and only with the approval of the president or a senior officer designated by the president.
A U.S. military spokesman in Baghdad refused to describe the current rules of engagement governing the use of riot-control agents.
Several Army officers who have served in Iraq say they have never seen riot-control agents used there by the U.S. military at all.