The Marines said the 16 Americans and three Iraqi contractors were arrested and held in a military jail for three days after spraying small arms fire at Iraqi civilians and U.S. forces from their cars in Fallujah late last month. There were no casualties.

Many Iraqis resent high-profile security details who speed along highways in sports utility vehicles bristling with automatic weapons. Senior government officials, who are prime targets of militants wreaking havoc across Iraq, use private security firms for their own protection.

No charges have been filed yet following the May 28 shootings.

Marines spokesman Lt. Col. Dave Lapan said Marines reported seeing gunmen in several late-model trucks fire "near civilian cars" and on military positions.

"Three hours later, another Marine observation post was fired on by gunmen from vehicles matching the description of those involved in the earlier attack," Lapan said.

U.S. forces later detained the contractors without incident and held them in a military jail for three days. The American contractors are thought to have left Iraq, the military said. A Naval Criminal Investigative Service inquiry is under way.

Iraq's rampant insecurity has spawned a thriving private security industry comprising Iraqis and former military personnel from military forces around the world to protect foreign contractors working on reconstruction projects, journalists and senior government officials and diplomats.

Meanwhile, the U.S. is talking with Sunni Arab leaders with ties to militant groups to try get them to lay down their arms, an American official said. Several groups are indicating a willingness to join the political process, but more radical militants can only be dealt with through military means, he said.

"In order to achieve stability and an end to the insurgency and stop Iraqis from being killed in large numbers, the insurgency has to be addressed," the Baghdad-based official said Wednesday during a briefing on condition he not be identified.

Top Sunni leaders also set themselves on a collision course Wednesday with the Iraqi government, demanding a greater say on a committee drafting the new constitution and threatening to boycott the process if they don't get more seats.

Sunni Arab support is crucial for Iraq's Shiite- and Kurdish-dominated government, particularly to approve the constitution. The draft charter will collapse if three of Iraq's four predominantly Sunni Arab provinces vote against it in a referendum later this year.

Separately, U.S. officials confirmed last week's arrest of Mullah Mahdi, Mosul cell leader of the feared Ansar al-Sunnah terror group, which has links to al-Qaida in Iraq. Iraqi and American forces have also captured numerous foreign fighters from Syria, Jordan, Saudi Arabia and Morocco recently.

In a related development, two Mahdi aides were captured Wednesday in Mosul, Asso Mamand, an official of the Patriotic Union of Kurdistan, said.

Gunmen killed former Baath Party member Kamil al-Nouri near his grocery store in Baghdad's impoverished Sadr City late Wednesday, police Maj. Hussein Jadou'a said. Al-Nouri was one of 10 Iraqis announced killed Wednesday across the country.