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An Al Qaeda leader, the head of a designated terror organization and a confessed jihadist-in-training are among a "Who's Who" of controversial figures who have participated in weekly prayer sessions on Capitol Hill since the 2001 terror attacks, an investigation by FoxNews.com reveals.

The Congressional Muslim Staff Association (CMSA) has held weekly Friday Jummah prayers for more than a decade, and guest preachers are often invited to lead the service. The group held prayers informally for about eight years before gaining official status in 2006 under the sponsorship of Rep. Keith Ellison, D-Minn., one of two Muslims currently serving in Congress. The second Muslim congressman, Rep. Andre Carson, D-Ind., joined as co-sponsor after he was elected in 2008.

Among those who FoxNews.com determined have attended the prayer services during the Clinton, George W. Bush and Obama administrations are:

Anwar al-Awlaki, the notorious Al Qaeda cleric believed to be hiding in Yemen and the lone American on the U.S. government’s capture or kill list, who conducted a prayer service on Capitol Hill shortly after the Sept. 11 terrorist attacks.

Randall “Ismail” Royer, a former communications associate for the Council on American-Islamic Relations (CAIR), who confessed in 2004 to receiving jihadist training in Pakistan. He is serving a 20-year prison term.

Anwar Hajjaj, former president of Taibah International Aid Association, which was designated a terrorist organization by the U.S. and U.N. in 2004.

Esam Omeish, the former president of the Muslim American Society, who was forced to resign from the Virginia Commission on Immigration in 2007 after calling for "the jihad way," among other remarks.

Salam Al-Marayati, president of the Muslim Public Affairs Council, who was forced to step down from a national terrorism committee post in 1999 for pro-terrorist comments.

Nihad Awad, CAIR executive director, who attended a Hamas meeting in Philadelphia in 1993 that was wiretapped by the FBI.

Johari Abdul Malik, Dar al-Hijrah imam, who made statements in support of convicted and suspected terrorists who attended his mosque.

Tariq Ramadan, a Muslim scholar banned from the U.S. for six years beginning in 2004 for his alleged ties and donations to terror groups. Secretary of State Hillary Clinton lifted Ramadan's ban in January.

—  Abdulaziz Othman Al-Twaijri, the head of a division of the Organization of the Islamic Conference, considered a foreign agent by the U.S.

It is unclear who else has attended these services, because there appears to be no public record of those CMSA has invited to Capitol Hill. The group’s website, included in the official congressional staff associations directory, displays an error message. And out of the more than 25 people associated with CMSA whom FoxNews.com contacted for this article — members, participants, guests, listed officers, congressional sponsors and Muslim advocacy groups — only one person was willing to speak.

“The U.S. Capitol ought to be one of the most transparent and public bodies, yet they get some public criticism about who they’re bringing in and they pull down their website," said Patrick Poole, an anti-terrorism consultant to law enforcement and the U.S. military who has written about CMSA for the conservative blog Pajamas Media.

"That’s not behavior conducive to people drawing public salaries and working in the halls of Congress,” Poole said.

But a portrait of the Jummah prayer meetings can be gleaned through video footage, news reports, court records and social media posts. And what emerges is a "Who’s Who" of controversial characters cycling through the doors of the Capitol on the invite of CMSA.

“There is a pattern of seriously bad actors not just being involved with, but leading this organization,” Poole said.

“There really needs to be some kind of investigation into who else CMSA is or has been bringing onto the Hill.”

The most notorious of the lot is al-Awlaki, who is seen leading the prayer service on Capitol Hill in video footage included in "MUHAMMAD: Life of a Prophet," a documentary that aired on PBS in 2002.

In the footage, Jameel Johnson, Rep. Gregory Meeks’ former chief of staff and founder and former president of CMSA, is seen sitting next to Randall “Ismail” Royer, a former communications associate for the Council on American-Islamic Relations (CAIR) who is now serving a 20-year sentence after pleading guilty in 2004 to helping jihadists from Virginia gain access to a terrorist training camp in Pakistan. That camp was run by Lashkar-e-Taiba, which was designated a terrorist organization by the United States in December 2001. One year before Royer was filmed attending the Awlaki prayer service, he attended jihad training camps in Pakistan, documents show.

Nihad Awad, executive director of CAIR, can also be seen at the Awlaki prayer session. Awad has spoken out in support of Hamas and attended a 1993 Hamas meeting in Philadelphia that was wiretapped by the FBI, according to public record and court documents from the Holy Land Foundation trial. CAIR was named as an unindicted co-conspirator in the trial.

Last year, the FBI severed ties with CAIR due to evidence of the group’s ties to networks supporting Hamas, which the State Department has designated as a terrorist group, according to documents obtained by the Investigative Project on Terrorism, a watchdog group.

"The FBI has had to limit its formal contact with CAIR field offices until certain issues are addressed by CAIR's national headquarters," FBI spokesman John Miller told FoxNews.com last year. "CAIR's leadership is aware of this. Beyond that, we have no further comment."

Awad's assistant, Asma Gheyoub, told FoxNews.com that she had passed along FoxNews.com's request for comment and that Awad would be in contact. But Awad has not responded.

Ibrahim Hooper, CAIR’s national communications director, also appears to have attended the Awlaki prayer service.

When asked by FoxNews.com if he had attended the service on Capitol Hill, Hooper said:

“Do you realize that the prayer services are open to the public, staffers, we have no say in who offers the prayer — or are you just trying to smear Muslims?”

When asked who chooses who offers the prayer, Hooper said, “You’d have to ask the staffers.”

When asked if he is in the video of the service or had attended any service on Capitol Hill led by Awlaki, Hooper said, “I don’t even remember. I don’t have a real big interest in furthering Fox News' anti-Muslim agenda, but thanks for calling.”

Sources told FoxNews.com that CMSA is comprised mostly of young Hill staffers who, for the most part, do not play a role in bringing in speakers; they say organizations like CAIR have a heavy hand in selecting and bringing in outside guests.

Ibrahim Ramey, human and civil rights director at Muslim American Society Freedom, a national Muslim advocacy group with a branch in Washington, told FoxNews.com that CMSA would never support anyone advocating violence against the U.S. Ramey said he's attended numerous CMSA functions and counts some of its members as friends.

"I do know that Sheikh Awlaki has declared war on America and American civilian targets and that he would not be welcome to participate in any CMSA program on Capitol Hill," Ramey said.

"He has made very clear statements about killing innocents, but he had been up until years ago, he had been a fairly respected member of the Northern Virginia Muslim community, where he would be now would not be compatible with the work we're doing at CMSA."

But video shot by Roll Call in April 2010 shows another set of controversial figures, including Anwar Hajjaj, who led the prayer.

To see Roll Call's video click here.

Hajjaj, tax filings show, was president of Taibah International Aid Association, which was designated as a terrorist organization by the U.S. Treasury Department in 2004 for its ties to a network funneling money to Hamas.

Hajjaj and Usama bin Laden’s nephew, Abdullah bin Laden, co-founded World Assembly of Muslim Youth, which the FBI has deemed a “suspected terrorist organization” since 1996, according to a complaint filed in New York federal court on behalf of the families of Sept. 11 victims. The judge refused to dismiss the charges against the World Assembly in September, saying the charges against it were "sufficient to demonstrate that they are knowingly and intentionally providing material support to Al Qaeda."

Hajjaj’s involvement with CMSA dates back at least to 2006, according to reports.

And there are others.

Esam Omeish, former president of the Muslim American Society, has led CMSA prayer services on Capitol Hill. He's called for "the jihad way," supported suicide bombers and advocated for the impeachment of President George W. Bush, according to reports and video.

According to State Department documents, “In 2007, he had to resign from the Virginia Commission on Immigration due to comments he made regarding “Israel’s war machine” during the 2006 Israel-Hezbollah conflict. He is also accused of supporting suicide attacks and violence in achieving a Palestinian state.”

Another controversial figure is Salam Al-Marayati, president of the Muslim Public Affairs Council, who spoke at a CMSA forum aired on CSPAN in August of this year. In 1999, Marayati was forced to step down from a national terrorism committee post after some inflammatory statements he'd made became public.

In a 2003 interview with the Los Angeles Times, Marayati said his organization's members were angry at "the FBI's policy of targeting people because of their race and religion."

"That's what they've been doing since the attacks, and we don't know of any case that has resulted in the arrest, indictment or prosecution of a terrorist," he said.

In 2006, at a fundraising dinner for Sami Al-Arian, who pleaded guilty to a charge of conspiring to provide services to the Palestinian Islamic Jihad, a designated terror group, Marayati said:

"So if we have this case where we are being dictated upon, not only on terminology, but dictated upon on who speaks for us, and our organizations, our charities, are shut down one by one. Therefore, brothers and sisters, there is a storm that is coming. That storm is going to be worse than Japanese internment."

In an interview in 2000, former FBI counterterrorism chief Steven Pomerantz said, “Mr. Marayati has justified and defended the activities of terrorist organizations such as Hamas, which, among other violent activities, has been involved in the murder of American citizens. He has also categorized the Hezbollah terrorist attack, which killed 241 U.S. Marines, as 'a military operation.'"

But Ramey denied that anyone would knowingly bring supporters of violence onto the Hill, calling the work of CMSA and violent jihadists mutually exclusive.

"I'm sure that there is no support for anything that is illegal or violent vis-a-vis the United States," he said.

"We repudiate those things — we believe social change is necessary but something we would want to accomplish legally, with above-board efforts."

The CMSA Twitter feed identifies other guests on Capitol Hill:

— Johari Abdul Malik, the imam of Dar al-Hijrah, the Falls Church, Va., mosque where Awlaki was once imam, has made statements in support of members of his mosque convicted on terror-related charges, according to reports and documents. Among them is Omar Abu Ali, who was convicted in 2005 of providing support to Al Qaeda and plotting to assassinate President George W. Bush.

Malik, who is on a State Department junket in Afghanistan, told FoxNews.com via e-mail that he was heading abroad and wouldn’t be able to comment until mid November.

— Tariq Ramadan, who was banned from the U.S. for six years for his alleged donations to Hamas, a group since classified by the U.S. Treasury Department as a terrorist organization. In April, CMSA’s Twitter account announced the group as “Honorary Hosts for a Capitol Hill welcome reception this afternoon for Professor Tariq Ramadan! He’s beginning his U.S. speaking tour.”

— Abdulaziz Othman Al-Twaijri, the head of the Islamic Educational, Scientific and Cultural Organization, a division of the Organization of Islamic Conference, attended a CMSA briefing in May at the Capitol Visitor Center.

“The Organization of the Islamic Conference — he’s a foreign agent,” Poole said. “That’s like having the Iranian government come in and speak on Capitol Hill.”

The Capitol Hill chaplain, the Rev. Daniel Coughlin, said there are numerous staff organizations on the Hill — the Black Caucus and the Golf Association, for example — but only a few official religious ones. And they, like the non-religious staff associations, are overseen by the Committee on House Administration.

There are also other ad hoc prayer groups that meet in an unofficial capacity.

"It's more than prayers," Coughlin said. "Each different religious group has different sessions, maybe they have speakers or a social."