The U.S. government had little doubt about who was behind the 1984 attack, even before crime-scene analysis and sensitive source reporting began to flow in. Writing just days after the second embassy bombing, the CIA noted that “an overwhelming body of circumstantial evidence points to the Hizb Allah, operating with Iranian support under the cover name of Islamic Jihad.” For one thing, the suicide vehicle bomb employed had become a trademark of the group. And, the CIA added at the time, “Shia fundamentalists are the only organized terrorists in Lebanon likely to willingly sacrifice their lives in such an attack.” Following the bombing, two callers claimed responsibility in the name of Hezbollah’s Islamic Jihad Organization (IJO). Several times in the year to follow, the CIA noted, anonymous callers in Beirut warned that the IJO planned to continue attacking U.S. interests. FBI forensic investigators determined that the marine barracks bombing was not only the deadliest terrorist attack then to have targeted Americans, it was also the single-largest non-nuclear explosion on earth since World War II. Composed of at least 18,000 pounds of explosives— the equivalent of six tons of dynamite— the bomb demolished the four-story building on the fringe of the Beirut Airport campus, leaving behind a crater at least 13 feet deep and 30 feet wide. So many marines, sailors, and soldiers perished that day that the base ran out of body bags. At the French MNF building, the deaths of 58 French paratroopers marked the French military’s highest death toll since the Algerian war ended in 1962. The eight-story building where the paratroopers were staying was literally upended by the blast.

Imad Mughniyeh, the Hezbollah operational leader and terrorist mastermind, and his brother-in-law and cousin, Mustapha Badreddine, reportedly not only watched the marine barracks bombing through binoculars from a perch atop a nearby building overlooking their neighborhood but also coordinated it. In February 1998, Lebanon’s highest court announced plans to try Hezbollah’s first secretary-general, Subhi al-Tufayli, for his role in the marine barracks bombing, among other crimes. At the time, the CIA assessed that Iran, Syria, and Hezbollah would likely help Tufayli escape so he could not “implicate them in a variety of illegal activities, including terrorist operations against U.S. citizens.” He was never tried. Another suspect was Mohammad Hussein Fadlallah, a leader of the Lebanese Shi’a community often described as one of Hezbollah’s founding spiritual figures.

In 1986, the CIA reported that Fadlallah “has long been recognized as the spiritual leader of and political spokesman for Lebanon’s Shia Hezbollah.” Fadlallah’s stature, the CIA added, grew “along with Hizballah’s political and military influence.” Fadlallah “benefited from and contributed to the growing extremism in the Shia community by his bold sermons attacking Israel and, later, the presence of the Multinational Force in Lebanon.” Lebanese Shi’a were inspired by the Iranian revolution to seek an Islamic state in Lebanon, and Fadlallah valued his ties to Iran, in large part because of the significant military, financial, and political assistance Tehran provided to Hezbollah. This assistance helped forge a powerful and potent militant Shi’a group out of several smaller groups.