U.S. intelligence officials believe that Iranian commandos took part in a deadly attack on a compound of dissidents inside Iraq and then spirited seven members of the group back to Iran, highlighting Tehran’s increasingly free hand inside Iraq in the wake of the U.S withdrawal from the country.

The Sept. 1 attack on a base called Camp Ashraf killed at least 50 members of the Mujahedeen-e Khalq, or MEK, which had disarmed at the request of the U.S. military after the American invasion of Iraq and received explicit promises of protection from senior commanders. Instead, gory videos released by the group showed that many of its members had been shot with their hands tied behind their backs or in one of the camp’s makeshift hospitals. MEK leaders, backed by an array of U.S. lawmakers, said Iraqi security forces carried out the attack.

Baghdad has long denied the charge, and U.S. officials have now concluded that a small number of Iranian paramilitaries from its feared Islamic Revolution Guards Corps helped plan and direct the assault on the camp. Three officials, speaking to Foreign Policy for the first time, said gunmen from two of Tehran’s Iraqi-based proxies, Kitab Hezbollah and Asaib Ahl al-Haq, then carried out the actual attack. The Iranian involvement in the Ashraf massacre hasn’t been reported before. […]