Speaking firms representing 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. The sources, all of whom spoke on condition of anonymity, said that speaking fees given to the former officials total hundreds of thousands of dollars.

"This is about finding out where the money is coming from," an Obama administration official familiar with the probe said. "This has been a source of enormous concern for a long time now. You have to ask the question, whether this is a prima facie case of material support for terrorism."

Freeh and Shelton are among 40 former senior U.S. government officials who have participated in a public lobbying campaign – including appearing at overseas conferences and speaking at public rallies – aimed at persuading the U.S. government to remove the MEK from the terror list.

First-class flights
Many of the speakers have received fees of about $30,000 or more per talk and first-class flights to European capitals, according to two sources familiar with the arrangements.

Edward Rendell, a former Pennsylvania governor and ex-Chairman of the Democratic National Committee, whose speaking firm also received a subpoena, has received $160,000 over the past year for appearing at about seven conferences and rallies, including some in Paris, Brussels and Geneva, according to his office. (Rendell is a contributor to MSNBC TV.)

The former officials have said they were told the fees came from wealthy American and foreign supporters of the MEK, not the group itself — and they resent any suggestion they are abetting a terrorist group.

"We’re all pretty miffed," Shelton told NBC News. "None of us involved in this would say a good word about anyone suspected of being a terrorist." But Shelton said that he’s "pretty passionate" that the MEK represents a legitimate resistance group fighting to overthrow "America’s number one enemy" — the Iranian government.

In a statement Friday, Hossein Abedini, a spokesman for the MEK,also denied the group has ever “paid senior former U.S. officials or any other dignitary in the U.S.”

“This is an utter lie and there is not even a scintilla of truth to it,” Abedini said. “The MEK, as the legitimate opposition to the clerical regime, enjoys international recognition in Europe and the U.S. The objective of this failed propaganda is to weaken the widespread public support of the members of Congress, officials and scores of U.S. generals for … revoking of the illegitimate and unjust terror listing of the MEK.”

Shelton said that he was informed by Keppler Speakers, the agency that handles his speaking engagements, that it had been subpoenaed for records of talks he has given over the past year at conferences and rallies sponsored by the MEK. He said Freeh told him that Greater Talent Network, the firm that handles the former FBI director’s speaking engagements, also received a subpoena.

Freeh did not respond to requests for comment. (A Keppler executive also did not respond. Reached by phone, Tom Marcosson, an executive vice president of Greater Talent, declined to comment.)

But Rendell told NBC News that he received an email this week from Freeh’s office alerting him and more than three dozen other former senior officials that subpoenas were being issued by the Treasury Department Office of Foreign Assets Control. The email asked that the former senior officials contact Freeh and former Attorney General Michael Mukasey. Freeh and Mukasey, who have been among the leaders in the campaign to "delist" the MEK, are hiring a lawyer to represent all former senior officials caught up in the investigation, the email from Freeh’s office said, according to Rendell.

John Sullivan, a spokesman for the Treasury Department, said the department does not comment on "potential" investigations. But he added in an email: "The MEK is a designated terrorist group, therefore U.S. persons are generally prohibited from engaging in transactions with or providing services to this group. The Treasury Department takes sanctions enforcement seriously and routinely investigates potential violations of sanctions law."

It is unclear how far Treasury Department officials intend to push the probe — or why they chose to launch it now, more than a year after the lobbying campaign began. But NBC News has obtained one possible clue: A small Pennsylvania-based speakers firm called Speakers Access wrote an email in September inviting a Washington based national security expert to speak at a conference in Geneva, Switzerland "on behalf of our client, National Council of Resistance of Iran, Foreign Affairs Committee." The National Council of Resistance is considered by the Treasury Department to be one of the "aliases" of the MEK and is itself designated as a terror group.

‘Mistake’
The email was later turned over to the FBI and other U.S. officials. The Speakers Access executive who wrote the email, who asked not to be identified, said the email was a "mistake" and that the client was actually another organization — "the Committee for Human Rights in Iran," which is not on the terror list but which has the same contact in Paris as the National Council of Resistance of Iran.

The executive said Speakers Access has since ceased any dealings with either group and turned over all its records on the matter after receiving a Treasury Department subpoena months ago.

The investigation comes at a time of intense internal debate about the MEK, in part spurred by assertions it could prove a useful ally in pressuring the Iranian government to suspend its nuclear program. NBC News reported recently that MEK operatives, trained by the Israeli Mossad, are believed by some U.S. intelligence officials to have been involved in the assassination of Iranian nuclear scientists — a report that the group has denied as "absolutely false."

Israel teams 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.

"The MEK has a crazy edge to it," said Michael Leiter, former director of the National Counter-Terrorism Center and an NBC News consultant. "It always struck me as a cult as much as a terrorist group."

But the group’s supporters say it has long since publicly renounced violence and that Rajavi has proclaimed the group’s adherence to democratic principles. "They want the mullahs out of Iran and they want to replace them with a constitution based on the Declaration of Independence," said Shelton.

The group has also generated sympathy over the plight of its followers at Camp Ashraf, a paramilitary camp on the Iran-Iraq border, where they have been detained – and until recently protected – by the U.S military since the 2003 invasion of Iraq. U.S. officials have been seeking the group’s cooperation to resettle the estimated 2,500 remaining MEK members at Camp Ashraf to a new facility near the Baghdad airport, where they can be processed by the United Nations as refugees and resettled elsewhere.

But the process has stalled – in part over disputes about the conditions of transfer – and MEK advocates say they fear the government of Iraqi Prime Minister Nouri al-Maliki, at Iran’s urging, may move in to slaughter the group’s members. "This could be a humanitarian disaster," said Rendell.

Rendell said that there have been weekly conference calls among a "core group" of former U.S. senior officials participating in the lobbying campaign, organized by Freeh, to talk about ways to prod the State Department to remove the MEK from the terror list and protect its followers at Camp Ashraf. He identified this group as including former Homeland Security Secretary Tom Ridge, former Democratic National Committee Chair Howard Dean and Mukasey — all of whom have publicly spoken out on behalf of the MEK and spoken at its rallies.

Officials act as middle men
These weekly conference calls have also turned into back channel negotiations over the Camp Ashraf issue. In recent weeks, Rendell said, State Department Ambassador Daniel Fried, the special envoy for detainee issues, has joined the phone calls, urging the pro-MEK "core" members to pass along messages to MEK leaders in Paris, Rendell said.

"The core group talks to Freeh every week," he said. "It’s Ridge, myself, Dean, Freeh, Mukasey. Shelton has joined us on occasion. … We were the ones that Fried asked to communicate with the MEK, telling them, ‘This is the best deal you’re going to get.’ He will say, ‘Listen, you guys have to persuade the MEK to do this. Tell them, OK, tell Paris, they have to persuade the people to get on the buses (at Camp Ashraf.) We then communicate [with the MEK]."

Fried declined comment. But a senior State Department official confirmed his participation in the calls as a means of communicating with MEK leaders in Paris — something U.S. officials are barred from doing — in order to work out a "peaceful" resolution over the conflict over Camp Ashraf.

Rendell said that he and other members of the core group have met with Rajavi in Paris and sent emails to her chief deputy, Farzin Hashemi, passing along Fried’s messages. "The bottom line is, we all believe we are protecting people," he said.

But the bottom line for some U.S. officials is that the former government officials participating in the pro-MEK campaign are being paid handsomely for promoting a dubious cause sponsored by an officially designated terrorist group. Despite the public lobbying campaign, there is still deep suspicion about the MEK and its motives — and concerns that once its members leave Camp Ashraf, many of its followers will return to terrorism, said one senior official speaking on condition of anonymity.

"It’s extraordinary that so many distinguished public servants would shill for a group that has American blood on its hands," the official said.

By Michael Isikoff , National investigative correspondent