American intelligence officials say they believe that since a handful of Iranian Revolutionary Guard operatives were captured in Baghdad in 2006, Iran shifted its strategy to bringing small groups of Iraqis into Iran. The Iraqis are then sent back to their country to train larger cadres of Shiite militants.

One senior American intelligence official said there were indications that the training programs in Iran might have significantly expanded this year to accommodate the scores of Iraqi militia fighters who fled Iraq during the Iraqi military’s campaigns in Basra and Baghdad.

Brian Fishman, director of research at the Combating Terrorism Center and a co-author of a new study about Iran’s political and military influence in Iraq, said that even though Iran was not in direct command of militia groups in Iraq, the training was one of the means at Iran’s disposal to increase or decrease its influence in Iraq at will.

“Having the militia allies is a hedge,” he said. “If things turn against Iran politically, it gives them a lever to pull.”

American officials say it is still murky just how much of a direct role senior Iranian officials take in the training, although they say they believe that it takes place with at least the tacit approval of elements of Iran’s government. The documents do not provide any direct evidence of senior Iranian government officials overseeing the training.

The new Iran study, written by Mr. Fishman and Col. Joseph H. Felter, concludes that Iran aims to attack American troops in Iraq in part to show off its own abilities and in part to “demonstrate a credible deterrent against a U.S. strike on Iran’s nuclear facilities.”

The captives gave detailed descriptions of daily routines in the Iranian camps, from the intensity of weapons training to the more mundane complaints of military life. One of the captured Iraqis described a mini-revolt among the trainees because they had not been issued socks to wear with their military boots.