CHICAGO, Aug. 25 - A prominent Muslim scholar from Switzerland was supposed to begin teaching a seminar on Islamic ethics at the University of Notre Dame on Tuesday, but he did not show up for his first class because the State Department revoked his visa.
University officials said an American diplomat telephoned the scholar, Tariq Ramadan, this month at his home in Geneva and told him that his permission to work in the United States, which was approved earlier this year, had been revoked. They said the diplomat offered no explanation.
On Wednesday, a spokesman for the Department of Homeland Security, Dean Boyd, said his agency had given the State Department information about Mr. Ramadan. He declined to say what it was.
"We provided the information to them, and they ultimately made a decision to revoke the visa," Mr. Boyd said. "Generally speaking, the criteria for revocation of visas include public safety risk or national security threat."
Kelly Shannon, a spokeswoman for the State Department, said Mr. Ramadan's visa was revoked under a legal provision that bans espionage agents, saboteurs and anyone the United States "knows, or has reasonable ground to believe, is engaged in or is likely to engage after entry in any terrorist activity." She said she could not provide any details about Mr. Ramadan's case.
Mr. Ramadan, 42, has written extensively about the challenges of blending Islam with Western habits and values. Some critics have interpreted his work as calling for conciliation between Muslims and Christians, but others have accused him of anti-Semitism and sympathy for terrorism.
In October a French newspaper, Le Parisien, reported that European intelligence agencies believed Mr. Ramadan had been in contact with leaders of Al Qaeda. Mr. Ramadan denied it.
A spokesman for Notre Dame, Matthew V. Storin, said the university had "vetted Mr. Ramadan very carefully." "We don't feel there is anything that should keep him out of the country, based on what we know," Mr. Storin said.
In Geneva, Mr. Ramadan said he hoped the United States would reconsider its decision. "If I really had something to hide, would really I be trying to go the States?" he asked in an interview with Reuters.
Mr. Ramadan had already sent his furniture to Indiana and enrolled his children in school there. Scott Appleby, director of the Joan B. Kroc Institute for International Peace Studies, where Mr. Ramadan was to teach at Notre Dame, said: "I worry about the implications for academic freedom and more generally for freedom of speech and the openness of American society. A secondary, more subtle level of concern is about how ready we are as a society to hear a discourse that is authentically Muslim, not an extremist discourse but one that is critical of U.S. policy."
To many Muslims, Mr. Ramadan's family background is at least as important as his writing. His grandfather, Hassan al-Banna, founded the militant Muslim Brotherhood in 1928 as an instrument for deposing Egypt's secular regime. The group no longer advocates violence, but its legacy is felt throughout the Arab world.
Last year Mr. Ramadan touched off a furor by accusing some French intellectuals of favoring Israel simply because they are Jewish. Mr. Ramadan told the French newspaper Le Monde that he would "entirely reject" any charge of anti-Semitism and asserted that he had had "no respite from combating all occurrences of anti-Semitism among Muslims."
Daniel Pipes, director of the pro-Israel advocacy group Middle East Forum, said groups in France approached American officials there and urged them to review Mr. Ramadan's status. He said the groups, which he declined to identify, "attempted to bring to the attention of the U. S. government who he really is."