Revealed: the steady flow of funds to members of Congress, lobbying firms and former officials in support of Iranian group

Supporters of a designated Iranian terrorist organisation have won a long struggle to see it unbanned in the US after pouring millions of dollars into an unprecedented campaign of political donations, hiring Washington lobby groups and payments to former top administration officials.

A Guardian investigation, drawing partly on data researched by the Centre for Responsive Politics, a group tracking the impact of money in US politics, has identified a steady flow of funds from key Iranian American organisations and their leaders into the campaign to have the People's Mojahedin Organisation of Iran removed from the list of terrorist organisations.

The US secretary of state, Hillary Clinton, is expected to notify Congress that the MEK will be removed from the terrorism list in the coming days.

The campaign to bury the MEK's bloody history of bombings and assassinations that killed American businessmen, Iranian politicians and thousands of civilians, and to portray it as a loyal US ally against the Islamic government in Tehran has seen large sums of money directed at three principal targets: members of Congress, Washington lobby groups and influential former officials.

Prominent among the members of Congress who have received fund is Ileana Ros-Lehtinen, the chair of the House of Representatives foreign affairs committee. She has accepted at least $20,000 in donations from Iranian American groups or their leaders to her political campaign fund.

Other recipients include Congressman Bob Filner, who was twice flown to address pro-MEK events in France and has pushed resolutions resolutions in the House of Representatives calling for the group to be unbanned. More than $14,000 in expenses for Filner's Paris trips were met by the head of an Iranian American group who also paid close to $1m to a Washington lobby firm working to get the MEK unbanned.

A Texas Congressman, Ted Poe, received thousands of dollars in donations from the head of a pro-MEK group in his state at a time when he was a regular speaker on behalf of its unbanning at events across the US, describing the organisation as the ticket to regime change in Iran.

Mike Rogers, chairman of the House of Representatives intelligence committee, has also received the backing of individuals and groups that support the unbanning of the MEK. Rogers has been among the strongest supporters in Congress of delisting the group, sponsoring resolutions and pressing other members of Congress to support the cause.

A leading advocate of unbanning the MEK and chairman of the foreign affairs committee's oversight subcommittee, congressman Dana Rohrabacher, has received thousands of dollars in donations from supporters of the banned group this year alone.

The Guardian sought comment from Ros-Lehtinen, Rogers, Filner, Poe and Rohrabacher. Only Rohrabacher responded.

He said he was comfortable accepting donations from MEK supporters but that the money has no influence on his position that it should be unbanned.

"I wouldn't doubt that people would donate to my campaign if it's something that they see as beneficial to them, to what they believe in, whether it's the MEK or whether it's anybody else," he said.

"The question is whether it's the right position to take or not and whether it's a benefit to the people of the United States as a whole. In this case I've no doubt that supporting the MEK under this brutal attack from the Mullah regime [in Tehran] is in the interests of what I believe in but also in the interests of the people of the United States."

Rohrabacher said the MEK's past attacks on Americans, its bombing campaign in Iran that killed top politicians and civilians, and its support of Saddam Hussein were history and the group has turned its back on violence. He also denied that public support for a designated terrorist organisation might put him in conflict with the law.

"This isn't a bad group. A long time ago, in their history, they certainly had a questionable time – 20, 30, 40 years ago. But I don't know of any evidence they've engaged in terrorism for many, many years," he said. "They're not a terrorist group simply because some bureaucrats in the state department say so."

Three top Washington lobby firms - DLA Piper; Akin Gump Strauss Hauer & Feld; and DiGenova & Toensing - have been paid a total of nearly $1.5 million over the past year to press the US administration and legislators to support the delisting of the MEK and protection for its members in camps in Iraq.

Two other lobby groups were hired for much smaller amounts. The firms employed former members of Congress to press their ex-colleagues on Capitol Hill to back the unbanning of the MEK.

Scores of former senior officials have been paid up to $40,000 to make speeches in support of the MEK's delisting. Those who have received money include the former chairman of the US joint chiefs of staff, General Hugh Shelton; ex-FBI director Louis Freeh; and Michael Mukasey, who as attorney general oversaw the prosecution of terrorism cases.

The former Pennsylvania governor, Ed Rendell, has accepted more than $150,000 in speaking fees at events in support of the MEK's unbanning. Clarence Page, a columnist for the Chicago Tribune, was paid $20,000 to speak at the rally. Part of the money has been paid through speakers bureaus on the US east coast.

Others accepted only travel costs, although in some cases that involved expensive trips to Europe.

In June, Newt Gingrich, the former speaker of the US House of Representatives and Republican presidential candidate, flew to Paris to address a pro-MEK rally and meet its co-leader, Maryam Rajavi. He was criticised for bowing to her.

Congressman Rohrabacher has described the lobbying campaign as one of the most effective he has seen on Capitol Hill. It has galvanised powerful support for delisting the MEK far beyond those receiving political contributions, lobbying fees or other payments.

Ros-Lehtinen has been a vigorous proponent of recognition of the MEK, flying around the country to speak in support of unbanning the group and pressing the issue among fellow members of Congress. She has accepted an award from one group funding the campaign to delist the MEK. Other recipients of political donations, including Rogers, Filner and Rohrabacher, have also lobbied other members of Congress to support the unbanning. As a result, nearly 100 members of Congress have co-sponsored a resolution demanding the Obama administration to delist the MEK.

Last month, 17 former senior officials and US generals called on the state department to remove the group's terrorist designation. Among them were General James Jones, Barack Obama's former national security adviser; Tom Ridge, the former homeland security director; as well as Mukasey, Freeh and Rendell.

Some of the same politicians and former officials have also targeted newspapers and online publications in a campaign of opinion articles and letters aimed at changing the image of the MEK as a terrorist group.

The campaign has in part been funded by substantial donations from Iranian Americans and a web of organisations they lead from Florida to Texas and California.

The most generous benefactors include:

• Saeid Ghaemi, head of Colorado's Iranian American Community, who paid close to $900,000 of his own money to a Washington lobby firm for its work to get the MEK unbanned.

• Ali Soudjani, president of the Iranian American Society of Texas. He gave close to $100,000 over the past five years to congressional campaign funds. His organisation paid more than $110,000 in fees to lobbyists last year.

• Ahmad Moeinimanesh, leader of the Iranian American Community of Northern California. The group paid $400,000 to a lobby firm. Moeinimanesh made personal donations to Ros-Lehtinen's campaign even though her constituency is several thousand miles from where he lives.

Some of the payments have prompted an investigation by the US treasury department. It is examining the fees paid to Shelton, Freeh, Mukasey and Rendell, and possibly others, to see if they breach laws against "material support for a terrorist group". In cases involving links to other banned organisations, such as Hamas and Hezbollah, individuals have received long jail sentences for indirect financial support.

The original source of the considerable sums involved is not always clear as groups making political donations or funding lobby firms are not required to declare their origin. Previously the MEK has relied in part on funding from Iraq under Saddam Hussein.

Soudjani told the Guardian that the moneys were raised from Iranian Americans in the US. "The Iranian community is wealthy. It has more than $600bn in the United States. This is pennies for supporting freedom," he said.

Asked if his own donations to members of Congress was specifically because of their positions on the MEK, he replied: "Yes, it is."

However, Soudjani was careful to say that the support is not for the MEK as an organisation, which could open donors to investigation under anti-terrorism laws.

"We are not giving material support to the MEK. We are supporting freedom of speech for justice and peace in Iran," he said.