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Minnesota Rep. Keith Ellison's groundbreaking pilgrimage to Mecca last month was paid for by an American Muslim organization that has ties to Islamic radicals and is "the Muslim equivalent of the neo-Nazi party," his critics say.

Ellison, a Democrat, became the first U.S. congressman ever to make the hajj pilgrimage when he visited Islam's holy city in December. The trip was funded by the Muslim American Society of Minnesota, a non-profit interfaith group that is one of 55 branches of the MAS nationwide. The pilgrimage was hailed by Muslim activists in the U.S.

"A U.S. congressman going on hajj sends a very positive message to the Muslim world about America and the religious diversity in America," said Ibrahim Hooper, communications director for the Council on American-Islamic Relations, a Muslim advocacy group.

But Ellison, the only Muslim in Congress, is coming under fire for his ties to MAS, which one terrorism expert called "the Muslim equivalent of the neo-Nazi party."

"It is the de facto arm of the Muslim Brotherhood in the U.S.," said Steven Emerson, director of the Investigative Project on Terrorism. "The agenda of the MAS is to ... impose Islamic law in the U.S., to undermine U.S. counterterrorism policy."

The MAS was founded by members of the Muslim Brotherhood, an international Islamist movement created in Egypt in 1928. Radical members of the Brotherhood founded the terror group Hamas and were among the first members of Al Qaeda.

The Muslim American Society's former secretary general has acknowledged that the group was founded by the Brotherhood, and in 2004 he estimated that about half of MAS members were in the Muslim Brotherhood.

"Ikhwan [Brotherhood] members founded MAS, but MAS went way beyond that point of conception," Shaker Elsayed told the Chicago Tribune, explaining that the group had expanded to include a wider viewpoint.

The Department of Justice has never taken action against MAS and declined to comment on whether it was investigating the group's ties.

"As a general rule, the Justice Department does not comment on whether or not a particular group or individual is under investigation or has been under investigation," said Dean Boyd, a spokesman for DOJ.

Ellison gave the keynote address at MAS-Minnesota's conventions in 2007 and 2008, and one of the organization's directors took time off his job to campaign for Ellison during his run for the House in 2006. Ellison was also the first guest on an MAS radio show launched last month.

Representatives for MAS-Minnesota did not return calls or e-mails requesting comment. But Mahdi Bray, the national director who campaigned for Ellison, denied his group had funded the congressman's pilgrimage, calling the story a "myth" and "urban legend" that needed to be laid to rest.

"Keith Ellison is a member of Congress who knows that congressmen don't take trips sponsored by nonprofits. That would be a breach of congressional ethics," said Bray, executive director of the MAS Freedom Foundation.

But Ellison's office told FOXNews.com that MAS had indeed paid for the congressman's hajj.

"The trip, funded by the Muslim American Society of Minnesota, was fully reviewed and approved in advance by the House Ethics Committee," Ellison's office said in a statement to FOXNews.com.

The office, which did not say how much Ellison's trip cost, defended MAS as a highly regarded interfaith organization with many ties in Minnesota.

"The Muslim American Society of Minnesota is a widely respected 501c3 organization incorporated in Minnesota, whose mission is to promote interfaith understanding," Ellison's office said in a statement to FOXNews.com. "The Muslim American Society of Minnesota (MASMN) has an ongoing interfaith project with the Minnesota Council of Churches, and a community outreach partnership with the St. Paul Police Department."

Ellison's office stressed that prominent Minnesota politicians, including former Vice President Walter Mondale, are close to the MAS. It did not address allegations that MAS has ties to the Muslim Brotherhood.

Emerson said Ellison's embrace of MAS was an attempt to legitimize the group, which he called out of the mainstream. "It's very troubling that he is trying to project an image of moderation, but he is tied to these radical groups," Emerson told FOXNews.com.

MAS has stirred up controversy in Minnesota for its activities in the state, including a fatwa, or religious edict, it issued barring Muslim taxi drivers from carrying passengers with alcohol. MAS said in its 2006 decree that Islamic jurisprudence prohibited carrying those passengers "because it involves cooperating in sin according to Islam."

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The MAS magazine, "The American Muslim," has printed articles defending "martyrdom operations" to wrest control of Gaza and the West Bank from Israel.

The Minnesota chapter at one time featured writings from Islamic clerics praising Hamas and urging Muslims to "wage Jihad until death." The postings, which were active as late as June 2007 -- one month after Ellison first addressed the group's convention -- have since been removed.

Though the MAS Web site has been washed of the religious texts that critics found offensive, Emerson says the group's ties to the Muslim Brotherhood remain intact.

"Their founding ideologues are all members of the Muslim Brotherhood," Emerson said. "I don't think you can get more radical in the United States without crossing the line to incitement."

Those accusations have some Muslim activists fuming.

"If you read the anti-Muslim hate sites on the Internet, every Muslim is the devil incarnate," said Hooper.

"In general any Muslim who seeks to have positive participation in American society is going to come under attack by those who seek to marginalize and exclude the American Muslim community," he said.