[Al Jazeera] There are thousands of reports of "foreign fighters" detained by US troops, but it is often extremely difficult to determine their identities. Many were arrested carrying false passports, or no paperwork at all; US forces routinely guess their nationalities based on accent, dress and mannerisms. Thus, while a vast number of foreigners are detained by the US military, only a small fraction are positively identified as coming from one country or another.

A review of the documents -- in which only the fighters with positive identification are counted -- suggests, not surprisingly, that Syria and Saudi Arabia were the main sources of foreign fighters for the Sunni insurgency. In 2004, at least 20 Syrian fighters were either detained or killed in Iraq. That number jumps to 55 in 2005, and then begins to decrease: 43 in 2006, 34 in 2007 (including several "facilitators"), 12 in 2008, and five in 2009. (These numbers, again, are obviously an underestimate: The US National Bureau of Economic Research published a paper in 2006 that counted at least 66 Syrian foreign fighters in an eight-month period in 2005.)

Still, the pattern reflected in the documents mirrors statements from US military officers: They criticised Syria for years for allowing smugglers to operate a "pipeline" of foreign fighters into Iraq, but began praising Syria in early 2008 for better co-operation. Admiral Gregory Smith told reporters in early 2008 that the flow of foreign fighters from Syria had more than halved since early 2007.

Saudi Arabia, Iraq's neighbour to the south, was also the source of a significant number of foreign fighters -- at least 58 of them were positively identified over the six-year period covered by the documents, plus seven high-level facilitators or financiers. More than 40 other fighters were also presumed to be Saudis, based on their accents or their names.

Fighters from Egypt, Yemen, Libya, Afghanistan, Pakistan, Lebanon, Jordan and Algeria are also mentioned in the documents. And there are a handful of fighters from Western countries: several from the United Kingdom, at least two from Canada, and at least three Americans.