A member of an extremist cell believed behind a suicide attack that killed more than 20 people - including three US Marines - has been arrested, the US military said Friday. US spokesmen said it was unclear if the suspect, who was not identified, was directly involved in planning the attack, which happened Thursday in the town of Karmah in Anbar province about 20 kilometers west of Baghdad. A suicide bomber reportedly dressed in a police uniform detonated an explosive belt during a meeting of tribal sheiks opposed to al-Qaida in Iraq. In addition to the Marines, two Iraqi interpreters, the local mayor and several key tribal figures were killed. The attack occurred two days before US officials planned to formally hand over security responsibility for Anbar to the Iraqis, marking a major milestone in the transformation of a province that had been the most violent in Iraq.