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.