Answer
Hezbollah's chief of operations Imad Mughniyeh is thought to have been responsible for most of the deaths of American citizens before the September 11 attacks. He is suspected of masterminding the bombings of the American Embassy and Marine Barracks in Beirut in 1983, which killed 241 US servicemen and civilians. Mughniyeh is also believed to have been involved in the kidnapping and murder of US journalist William Buckley in 1985.