Scholars and critics worldwide are demanding that the U.S. government explain why it revoked the work visa of a Muslim scholar hired at the University of Notre Dame (search), saying the action threatens academic freedoms.

But few answers are forthcoming from the Department of Homeland Security (search), which cited security concerns when it barred Tariq Ramadan (search) from entering the country.

That silence has sparked protests from at least four U.S. scholars' groups, led a United Nations-sponsored institution to issue an academic freedom alert and inspired appeals from Jewish organizations.

Robert O'Neil, who is chairman of an academic freedom committee for the American Association of University Professors (search), said Ramadan's case could have a chilling effect on an academic community already facing security measures stemming from the 2001 terrorist attacks.

"It does suggest ... foreign scholars may be scrutinized more carefully and may be denied entry on the basis of something less than overt terrorist activity or association," said O'Neil, whose group has written Secretary of State Colin Powell and Homeland Security Secretary Tom Ridge to protest the decision.

The State Department issued Ramadan a work visa in May but revoked it in July. The action came just weeks before the scholar was scheduled to begin a tenured position as professor of religion, conflict and peace-building at Notre Dame's Kroc Institute for International Peace Studies (search).

Russ Knocke, a Department of Homeland Security spokesman, said last month that the visa was revoked based on "public safety or national security interests." Knocke has not responded to recent requests for interviews from The Associated Press.

Many who have rallied in Ramadan's support believe the scholar's controversial profile, including sharp criticism of Israel, the war in Iraq and U.S. policy in the Middle East, was the real reason for the revocation.

"We fear that pressures were applied to reverse the granting of the visa by people who disagree with Dr. Ramadan's views as a scholar," two groups, the Middle East Studies Association of North America (search) and the American Academy of Religion (search), stated in a joint letter to Powell and Ridge.

Scholars at Risk (search), which normally focuses on rescuing professors who face persecution in their homelands, also has taken up Ramadan's cause because of its ramifications for academic freedom.

"The information we do have does not seem to explain the decision, and if there is more, we'd like to know what it is," said Robert Quinn, director of the group, which is based at New York University.

The Network for Education and Academic Rights (search) issued an academic freedom alert for the United States over the case. It is the fifth alert the London-based, U.N.-sponsored group has issued for the United States since January 2002.

The Jewish Law Students Society at Notre Dame (search) condemned the visa revocation. Chicago's Jewish Council on Urban Affairs (search) said "the barring of Ramadan may represent one more horrific example of government suspicion, intimidation and exaggerated allegations against Muslims and Muslim communities."

Notre Dame spokesman Matt Storin said the university is still seeking answers but has received no specifics about Ramadan's case.

Ramadan remains in Switzerland, defending himself in articles and interviews.

The furor over his visa symbolizes something larger, he told The Associated Press in a telephone interview.

"All these people who don't know me, they may disagree with me, all the academics, but at least they understand something: In Tariq Ramadan there is a very important challenge, a very important issue, which is the freedom for the academics to speak and to be critical, to be free."