Perpetrators of the worst-ever terror attack are yet to be identified, but with the name of Osama bin Laden constantly in the news, US Muslims and Arab-Americans are bracing for a backlash.
Within hours of the attacks, vitriol started flowing in Internet chat rooms, and around the country anonymous threats were telephoned to mosques and taunts were made to Arab-owned businesses.
At the Arab-American Institute in Washington, operators fielded dozens of phone calls from people making threatening statements and telling the group it was unwelcome in the US, said Jenny Salan, the group's media director.
The American-Arab Anti-Discrimination Committee in Washington, the Muslim Public Affairs Council in Los Angeles and the Islamic Networks Group in San Jose also reported calls with death threats, obscenities and racial slurs.
In New York, mayor Rudolph Giuliani said that neighborhoods with large Arab-American populations would receive extra police protection.
In other cities, such incidents were already striking fear into many of the country's estimated seven million Arab-Americans.
A Palestinian grocer in San Francisco said that he had endured taunts like "Go back to your country" and "We should kill you all."
"You can feel it in the air," said Souleiman Ghali, president of the San Francisco Islamic Society. "All the Arabs and Muslims in this country are beginning to feel suspect -- no matter how loyal they are." Ghali said that comparisons with Pearl Harbor were especially alarming. "I instantly thought of the Japanese internment camps," he said.
Slurs were especially harsh in Internet chat rooms.
"Now maybe all decent people will realize ... that the sub-human Arab maggot scum must be dealt with," read a message in an online discussion of Middle East politics.
A posting to an America Online chat room said, "Boycott Arabian Owned Businesses," while another wrote, "KILL ALL ARABS."
The rhetoric came despite blanket condemnations by American Muslim groups.
High government officials have said they strongly suspect bin Laden of masterminding Tuesday's attacks. Early reports suggest that the likely hijackers were Arab.
But similar suspicions existed after the 1995 bombing of the Oklahoma City federal building, until domestic terrorist Timothy McVeigh was arrested, tried, convicted and, on June 11, executed.
"Nobody knows who did it," said Suhaib Webb, imam of an Oklahoma City mosque. "Let's remember the Oklahoma City bombing, and when we rushed to judgement and accused Middle Eastern people. As a Muslim right now, we are very concerned. When you see the things you see on television, it is devastating. I think it should be known that there were Muslims in those buildings."
The Washington-based Council on American-Islamic Relations urged Muslim leaders nationwide to request extra police protection for mosques and advised believers who wear traditional Muslim garb to avoid public spaces for now.
"We have friends and family who work in that building, the World Trade Center. We have family and friends that worked at the Pentagon," said James Zogby, president of the Arab American Institute. "While we would like to mourn like everybody else in America, we end up looking over our shoulder because someone is pointing a finger."