To many academics, one of the most fearsome parts of the Patriot Act is section 411, which allows the government to deny visas to prominent individuals from abroad who have used their positions to endorse terrorism.

Many have feared that the provision is being used inappropriately against people who have not encouraged terrorism. But because the government doesn't release details on why it blocks some visas, it has been difficult for anyone to figure out whether visas have been improperly denied.

In an effort to find out exactly why such denials are taking place, three groups on Thursday sued the U.S. government, seeking to force various agencies to release documents related to the visa denials of several scholars. The groups filed Freedom of Information Act requests about the denials months ago, and sued after receiving practically no information.

"Our concern about academic freedom extends beyond the rights that are assured on our college and university campuses," Jane Buck, president of the American Association of University Professors, said in a statement. "We believe that the people of this country should be able to hear or read ideas from any speaker or writer without our government's restricting our access to a full range of perspectives. Indeed, the government of a free people is obliged to guarantee such access."

The AAUP joined in the suit with the American Civil Liberties Union and the PEN American Center. Officials of the Department of Homeland Security and other agencies did not return calls about the suit Thursday, but in the past have defended the Patriot Act and the secrecy surrounding its enforcement as necessary for national security.

Ruth Flower, a spokeswoman for the AAUP, said that the groups seeking the documents view obtaining the paper as a step toward a larger goal of making sure that scholars can visit the United States. "If there is a clear pattern of abuse or illegal action found, I imagine the parties would be interested in further steps," she said.

She also said she suspected that there were more cases of scholars being denied visas than those cited in the suit. The most prominent scholar named is Tariq Ramadan, who is Swiss and is considered one of the world's leading scholars on Islam. Last year, U.S. authorities revoked a visa for Ramadan, who was to have been a visiting professor at the University of Notre Dame, and scholars there and elsewhere scoffed at the idea that he could have posed any threat to Americans.

"If this can happen with him, the question is who else is it happening to," Flower said.

Other cases cited in the suit include those of Dora Maria Tellez, a scholar from Nicaragua who had to turn down a teaching position at Harvard University, and several delegations of Cuban scholars who were denied visas to attend scholarly meetings in the United States.