A Sunni mosque was leveled by an explosion Saturday in Basra, police and witnesses said - the second such attack in as many days, in apparent retaliation for the toppling of minarets at a prized Shi'ite shrine in Samarra. Bombers loaded into pickup trucks pulled up to the al-Ashrah al-Mubashra mosque in Basra's al-Hakimiya district at dawn, residents in nearby houses said. Minutes after they left, a huge explosion tore through the building, leveling it completely. It was unclear whether there were any guards present at the time, and why Iraqi security forces were unable to intervene. Witnesses said they saw no sign of police responding. As they were leaving, the insurgents wrote graffiti on the mosque complex's outer wall with the names of revered Shi'ite saints, witnesses said. They also hoisted a green Shi'ite flag over a crumbling part of the mosque complex, they said. Some nearby houses were damaged in the blast, but no injuries were reported. Basra is Iraq's second-largest city, 550 kilometers southeast of Baghdad.