Iraqi officials say a suicide car bomb exploded at a police checkpoint in the northern city of Mosul, killing at least two policemen. The blast Wednesday comes as Iraqi security forces press ahead with an offensive against al-Qaida in Iraq in Mosul - the group's last urban stronghold. In a separate attack, gunmen killed a Sunni sheik, his wife and two sons near Samarra. A police official says the sheik and one son were leaders in the anti-insurgent Awakening Council movement. All officials spoke on condition of anonymity because they were not authorized to release information.