About four out of every five detainees in American detention centers are Sunni Arab, even though Sunni Arabs make up just one-fifth of Iraq’s population. All of the foreign fighters listed on the materials found near Sinjar, excluding two from France, also came from countries that are predominantly Sunni.

Over the years, the Syrian border has been the principal entry point into Iraq for foreign insurgents, officials say. Many had come through Anbar Province, in west-central Iraq. But with the Sunni tribal revolt against extremist militants that began last year in Anbar, Al Qaeda in Mesopotamia and other jihadists concentrated their smuggling efforts on the area north of the Euphrates River along the Syrian border, the officials said.

The officials added that, based on the captured documents and other intelligence, they believe that the Sinjar cell that was raided in September was responsible for the smuggling of foreign fighters along a stretch of the border from Qaim, in Anbar, almost to the border with Turkey, a length of nearly 200 miles. They said that was why they were confident that the cell was responsible for such a large portion of the incoming foreign fighters.

American military and diplomatic officials who discussed the flow of fighters from Saudi Arabia were careful to draw a distinction between the Saudi government and the charities and individuals who they said encouraged young Saudi men to fight in Iraq. After United States officials put pressure on Saudi leaders in the summer, the Saudi government took some steps that have begun to curb the flow of fighters, the officials said.

Yet the senior American military officials said they also believed that Saudi citizens provided the majority of financing for Al Qaeda in Mesopotamia. “They don’t want to see the Shias come to dominate in Iraq,” one American official said.

The Sinjar materials showed that 291 fighters, or about 39 percent, came from North African nations during the period beginning in August 2006. That is far higher than previous military estimates of 10 to 13 percent from North Africa. The largest foreign fighter hometown was Darnah, Libya, which supplied 50 fighters.

For years American officials included Libya on the list of state sponsors of terrorism. But last year the United States removed it from that list and re-established full diplomatic relations, citing what Secretary of State Condoleezza Rice described as Libya’s “continued commitment to its renunciation of terrorism and the excellent cooperation” it has provided in the antiterrorism fight.