A new wave of car bombs rocked commercial streets in the Iraqi capital on Tuesday, killing 20 in the latest apparent attack by hard-line Sunni insurgents aiming to undermine confidence in the Shiite-led government. Meanwhile, Sunni leaders in Basra said unknown gunmen had shot dead 17 Sunnis in the Shiite-dominated city over the past two weeks, following threats to retaliate against them for attacks on Shiites in other parts of Iraq.