Iraq officials say three leading members of an anti-insurgent Sunni militia are in custody on suspicion of staging attacks before they switched sides to join US-led forces.
Iraqi officials say they are accused of violence during 2005 and 2006, including shooting down an American helicopter and targeting police.
Such arrests test Iraqi government pledges to reward so-called Awakening Council fighters, who turned against insurgents in 2006.
A local Awakening leader, Mullah Nadhum al-Jubouri, and his two brothers were detained in a raid on Saturday north of Baghdad. Officials say the suspects are being held Sunday at a US-Iraq base.
The officials spoke on condition of anonymity because they were not allowed to release the information.