A bomb explosion occurred at the shrine of Ali al-Ridha, the eighth Imam of Shia, on 20 June 1994 in a crowded prayer hall in Mashhad, Iran. To maximize the number of casualties, the explosion took place on Ashura, one of the holiest days for Shia muslims, when hundreds of pilgrims had gathered to commemorate the death of their third Imam, Husayn ibn Ali.The attack left at least 25 dead and at least 70 injured. The bomb was equivalent to 10 pounds of TNT, according to experts. Although a Sunni group claimed responsibility, the Iranian government laid the blame on the People's Mujahedin of Iran, and others have accused a Pakistani militant.