Aslam Abdullah, vice chairman of the Muslim Public Affairs Council in Los Angeles, says his group received a number of threatening phone calls and e-mails on Tuesday from unidentified people. "Basically their message is the same: Pack up and leave. You have no place in America," Abdullah says.

Jean Abinader of the Arab American Institute monitored talk radio from the public policy group's Washington, d.c., office. "(Tuesday) morning I was hearing people say, 'That's what we get for letting people like that into our country,'" he says.

Riffat Hassan, a professor of Islamic studies at the University of Louisville, says most of her Muslim students did not come to class Tuesday, including a woman who wears an Islamic head covering. "She was not even allowed to come out of her house," says Hassan, who received an e-mail from the student.

"We see a very bad backlash coming," says Abinader, "no matter who is responsible—even if it comes out that they are right-wing Americans, like Oklahoma City; still the reaction will be there."

In the days after the 1995 attack on the Murrah Federal Building in Oklahoma City, Muslims reported more than 200 incidents of harassment, threats, and violence, according to the Council on American-Islamic Relations (cair) in Washington, D.C. On Tuesday cair advised Muslims who wear Islamic attire to stay out of public areas "for the immediate future" and requested that authorities provide additional police patrols in the vicinity of mosques. CAIR estimates there are 7 million Muslims in the United States and ...