Seven mortar shells fell on the Shiite town of Musayyib, south of Baghdad, on Thursday, killing 21 people and wounding 65, Iraqi officials said. Security officials said the shells had been fired from a Sunni area near Musayyib where security forces were conducting a weeklong search for fighters from the militant group the Islamic State of Iraq and Syria. No group immediately claimed responsibility for Thursday’s attack on the residential area. The Sunni area, Jurf al-Sakhar, was one of Al Qaeda’s strongholds during some of the worst of the sectarian fighting in 2006 and 2007. Now members of the Islamic State of Iraq and Syria have moved to the area, according to officials, prompting the sweeps.