A large explosion Wednesday heavily damaged the golden dome of one of Iraq's most famous Shiite religious shrines, sending protesters pouring into the streets. It was the third major attack against Shi'ite targets in as many days. Police believed some people may be buried under the debris after the 6:55 a.m. explosion at the Askariya mosque but there were no confirmed figures. The shrine contains the tombs of two revered Shiite imams, both descendants of the Prophet Muhammad, and is among Iraq's most sacred sites for Shi'ite Muslims. Tradition says the shrine, which draws Shi'ite pilgrims from throughout the Islamic world, is near the place where the last of the 12 Shiite imams, Mohammed al-Mahdi, disappeared. Al-Mahdi, known as the "hidden imam," was the son and grandson of the two imams buried in the Askariya shrine. Shi'ites believe he is still alive and will return to restore justice to humanity. An attack at such an important religious shrine would constitute a grave assault on Shi'ite Islam at a time of rising sectarian tensions in Iraq. National Security Adviser Mouwafak al-Rubaie said armed men wearing special forces uniforms broke inside the shrine and seized the guards, including policemen, responsible for protecting the site. The gunmen planted the explosives and fled. Al-Rubaie blamed extremists represented by the al-Qaida terror network and Sunni militant group Ansar al-Sunnah for the explosion, which he told the Al Arabiya television aimed "to pull Iraq toward civil war."