An al-Qaida front group in Iraq has claimed responsibility for the wave of terrifying attacks that rocked Baghdad last week and killed 69 people. The coordinated attacks on Thursday targeted markets, grocery stores, cafes and government buildings in a dozen mostly Shiite neighborhoods. They coincided with a government crisis that has already strained ties between Sunnis and Shiites to the breaking point. They were the first major attacks since U.S. troops completed a full withdrawal from Iraq this month, increasing doubts about the ability of Iraqi security forces to protect the country. Nearly 200 people were wounded in the bombings. According to the SITE Intelligence Group, a U.S.-based organization that monitors jihadist Web traffic, the claim of responsibility was posted late Monday on militant websites.