Ex-FBI Director Louis Freeh and former chairman of the Joint Chiefs of Staff Gen. Hugh Shelton have received federal subpoenas as part of an expanding investigation into the source of payments to former top government officials who have publicly advocated removing an Iranian dissident group from the State Department list of terrorist groups, three sources familiar with the investigation told NBC News. The investigation, being conducted by the Treasury Department, is focused on whether the former officials may have received funding, directly or indirectly, from the People's Mujahedin of Iran, or MEK, thereby violating longstanding federal law barring financial dealings with terrorist groups. .

Israel teamed with terror group to kill Iran's nuclear scientists, U.S. officials say U.S. officials say that the MEK has a long history of terrorist acts, including bombings and assassinations, against Iranian leaders during the 1980s and that at least six Americans died in such attacks. The group — which was once allied with Iraqi dictator Saddam Hussein — is also viewed warily because of the slavish devotion of its followers to its Paris based leader, Maryan Rajavi.

Bush administration secretly trained an Iranian opposition group on the State Department's list of foreign terrorists. Hersh reports the U.S. Joint Special Operations Command trained operatives from Mujahideen-e-Khalq, or MEK, at a secret site in Nevada beginning in 2005. Hersh also discusses the role of Israeli intelligence and notes the Obama administration knew about the training in terms of the MEK and in terms of operations inside Iran.