The helicopter was hovering over a Baghdad checkpoint into the Green Zone, one typically crowded with cars, Iraqi civilians and United States military personnel.
Suddenly, on that May day in 2005, the copter dropped CS gas, a riot-control substance the American 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 American 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 United States military, but were part of a convoy operated by Blackwater Worldwide, the private security contractor that is under scrutiny for its role in a series of violent episodes in Iraq, including a September shooting in downtown Baghdad that left 17 Iraqis dead.
None of the American soldiers exposed to the chemical, which is similar to tear gas, required medical attention, and it is not clear if any Iraqis did. Still, the previously undisclosed incident has raised significant 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 American 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 Third Infantry Division who were involved in the episode said there were no signs of violence at the checkpoint. Instead, they said, the Blackwater convoy appeared to be stuck in traffic and may 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," Ms. 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 that the episode was reported to the United States Embassy in Baghdad, and that the embassy's chief security officer and the Department of Defense 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.
About 20 to 25 American soldiers were at the checkpoint at the time of the incident, and at least 10 were exposed to the CS gas after "rotor wash" from the hovering helicopter pushed it toward them, according to officers who were there. A number of Iraqi civilians, both on foot and in cars waiting to go through the checkpoint, were also exposed. The gas can cause burning and watering eyes, skin irritation and coughing and difficulty breathing. Nausea and vomiting can also result.
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 United States, although a 1975 presidential order allows their use by the United States military in war zones under limited defensive circumstances and only with the approval of the president or a senior officer designated by the president.
"It is not allowed as a method or means of warfare," said Michael Schmitt, professor of international law at the Naval War College in Newport, R.I. "There are very, very strict restrictions on the use of CS gas in a war zone."
In 2003, President Bush approved the use of riot control agents by the military in Iraq under the 1975 order, but only for such purposes as controlling rioting prisoners. At the time of Mr. Bush's decision, there were also concerns that the Iraqi Army would use civilians as shields, particularly in a last-ditch battle in Baghdad, and some officials believed that riot control agents might be effective in such circumstances to reduce casualties.
A United States military spokesman in Baghdad refused to describe the current rules of engagement governing the use of riot control agents, but former Army lawyers say their use requires the approval of the military's most senior commanders. "You never had a soldier with the authority to do it on his own," said Thomas J. Romig, a retired major general who served as the chief judge advocate general of the United States Army from 2001 to 2005 and is now the dean of the Washburn School of Law in Topeka, Kan.
Several Army officers who have served in Iraq say they have never seen riot control agents used there by the United States military at all. Col. Robert Roth, commander of Task Force 4-64 AR of the Third Infantry Division, which was manning the Assassins' Gate checkpoint at the time of the Blackwater incident, said that his troops were not issued any of the chemicals.
"We didn't even possess any kind of riot control agents, and we couldn't employ them if we wanted to," said Colonel Roth, who is now serving in South Korea.
But the same tight controls apparently did not apply to Blackwater at the time of the incident. The company initially got a contract to provide security for American officials in Iraq with the Coalition Provisional Authority, an agreement which did not address the use of CS gas. After the authority went out of business, the State Department extended the contract for another year until rebidding it. Blackwater and two other companies - DynCorp and Triple Canopy - that now provide security are not permitted to use CS gas under their current contracts, the State Department said.
The State Department said that its lawyers did not believe the Blackwater incident violated any treaty agreements.
In a written statement, the State Department said the international chemical weapons convention "allows for the use of riot control agents, such as CS, where they are not used as a method of warfare. The use of a riot control agent near a checkpoint at an intersection in the circumstances described is not considered to be a method of warfare."
Yet experts said that the legal status was not so clear cut. "I have never seen anything that would make it permissible to use tear gas to get traffic out of the way," Mr. Schmitt said. "In my view, it's an improper use of a riot control agent."
Blackwater's regular use of smoke canisters, which create clouds intended to impede attacks on convoys, also sets it apart from the military. While it does not raise the same legal issues as the CS gas, military officials said the practice raised policy concerns. Col. Roth said that he and other military officers frowned on the use of smoke, because it could be used for propaganda purposes to convince Iraqis that the United States was using chemical weapons.
Officers and soldiers who were hit by the CS gas, some of whom asked not to be identified because they were not authorized to discuss the incident, have described it with frustration. They said no weapons were being fired or any other violence that might have justified Blackwater's response.
In a personal journal posted online the day of the incident, Captain Clark provided a detailed description of what happened and included photos.
While standing at the checkpoint, he wrote, he saw a Blackwater helicopter overhead.
"We noticed that one of them was hovering right over the intersection in front of our checkpoint," he wrote. "There was a small amount of white smoke coming up from the intersection. I grabbed my radio and asked one of the guard towers what the smoke was. He answered that it looked like one of the helicopters dropped a smoke grenade on the cars in the intersection. I asked him why were they doing that, was there something going on in the intersection that would cause them to do this. He said, nope, couldn't see anything. Then I said, well what kind of smoke is it?
"Before he could say anything, I got my answer. My eyes started watering, my nose started burning and my face started to heat up. CS! I heard the lieutenant say, "Sir that's not smoke, it's CS gas."
After reporting the incident to his superiors, Captain Clark wrote, a convoy that the helicopter was protecting showed up. Because the gas caused a "complete traffic jam in front of our checkpoint," the captain wrote, "armored cars in the convoy made a U-turn - and threw another CS grenade."
"It just seemed incredibly stupid," he wrote. "The only thing we could figure out was for some reason, one of them figured that CS would somehow clear traffic. Why someone would think a substance that makes your eyes water, nose burn and face hurt would make a driver do anything other than stop is beyond me."
Army Staff Sgt. Kenny Mattingly also was puzzled. "We saw the Little Bird (Blackwater helicopter) come and hover right in front of the gate, and I saw one of the guys dropping a canister," Sergeant Mattingly said in an interview. "There was no reason for dropping the CS gas. We didn't hear any gunfire or anything. There was no incident under way."
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