The FBI called Adnan G. El Shukrijumah, 27, an "imminent threat to U.S. citizens and interests" who is "suspected of planning terrorist activities." A senior law enforcement official described him as a possible terrorism organizer in the style of Mohamed Atta, the suspected ringleader of the Sept. 11, 2001, attacks. But authorities said they have no details on what kind of plot he might be involved in.

An alias used by El Shukrijumah "kept coming up in numerous places," including interrogations of captured al Qaeda lieutenant Khalid Sheik Mohammed, an official said. El Shukrijumah is believed to have a connection, as yet unclear, to Jose Padilla, the American al Qaeda suspect held on charges he was plotting to explode a radiological bomb in the United States.

U.S. authorities also recovered a document that links the same alias to the Oklahoma flight school where Zacarias Moussaoui, the only person in the United States charged as a conspirator in the Sept. 11, 2001, attacks, studied aviation, an official said. There is no evidence that El Shukrijumah received pilot training in the United States, the official said.

As the U.S. invasion of Iraq began yesterday, FBI agents began interviewing about 11,000 Iraqi nationals around the country in search of intelligence tips, while officers from the Bureau of Immigration and Customs Enforcement arrested others who were in violation of immigration laws. Authorities refused to reveal how many Iraqi nationals were taken into custody, but said they were concentrated among sizable Iraqi communities in Detroit, Los Angeles, Miami, Houston, Boston and San Diego, an immigration official said.

"The joint initiative . . . is aimed at taking individuals off the street who might pose a threat to the safety and security of the American people," the immigration bureau said in a statement. "The Iraqis targeted as part of the effort were identified using a range of intelligence criteria, and all are in the country illegally."

On a day when French officials discovered trace amounts of the poison ricin in vials at a luggage facility in a Paris train station, U.S. health officials began shipping antidotes for nerve agents and cyanide to large emergency rescue squads around the country.

Homeland Security Secretary Tom Ridge, testifying before a House appropriations panel yesterday, said the department had not taken any steps to counter terrorism after the start of the war against Iraq on Thursday. The administration this week raised the national threat index to orange, indicating a "high" risk of attack, and implemented operation "Liberty Shield," increasing security at airports, public buildings and transportation hubs with state and local officials, he said.

El Shukrijumah may be traveling on passports from Guyana, Trinidad, Canada or Saudi Arabia, the FBI said. He last entered the United States before the terrorist attacks in New York and on the Pentagon and left later that year, a law enforcement official said. An official said that although authorities do not know where he is, there are indications he is in Morocco. But authorities fear he may have reentered the United States illegally.

The El Shukrijumah family moved to Miramar, a suburb north of Miami, in 1995, according to Neville and Una Khan, who live in the same neighborhood and have known the family since the 1960s. The Khans said that El Shukrijumah's father is a prominent Muslim leader in this suburb north of Miami and is the head of a prayer center, Masjid al Hijrah, next door to the family home. That home was searched yesterday by FBI agents.

Una Khan described Adnan El Shukrijumah as a devout student of the Koran who worked with children at the prayer center. He tended to be modern in his thinking, she said. "He never indicated in any way that he was extremist. . . . This is such a shock; this is something I don't understand. I can't believe it."

The Khans say they have not seen El Shukrijumah for several years. They believe he is doing Islamic missionary work, though they don't know where. He was also earning money by selling Islamic books, they said.

Several officials said the FBI decided to issue a public alert because of El Shukrijumah's tentative connections to Padilla and Moussaoui, the apparent references to him in terrorism-related documents and interrogations, and because they were unable to find him. One of El Shukrijumah's recent addresses was a house in Pembroke Pines, Fla., where Padilla attended a mosque.

The public plea for information came after weeks of investigation that centered on an alias of El Shukrijumah that the FBI had attached to another man, a law enforcement official said. The bureau yesterday rescinded a February alert issued under that man's name, Mohammed Sher Mohammed Khan. That earlier search was one factor that led to the elevation of the U.S. threat level last month.

"El Shukrijumah is possibly involved with al Qaeda terrorist activities and, if true, poses a serious threat to U.S. citizens and interests worldwide," the FBI said in a news release yesterday.

The FBI interviews and scattered arrests of Iraqi nationals yesterday by agents and immigration officers prompted widespread concern among Iraqi and Arab American groups. Authorities stressed that the efforts were aimed primarily at enlisting help in the war effort.

The FBI plans to interview 460 Iraqis in the Washington and Baltimore areas. In Michigan, home to the nation's largest concentration of Iraqi nationals, about 400 Iraqis are expected to be asked to participate in interviews. "We want to do some intelligence-gathering regarding the military effort in Iraq and some intelligence information that the FBI is looking for," FBI spokeswoman Dawn Clenney said. "These interviews are strictly voluntary, nonconfrontational."

Lavinia Limon, executive director of Immigration and Refugee Services of America, said an Iraqi-born refugee who is a U.S. citizen living in the Washington area told her that two FBI agents had interviewed him at his house for about 20 minutes Wednesday morning. According to the account, the agents wanted to know his views on the war with Iraq and whether he knew of anyone who might be planning attacks on the United States.

"The agents were very professional and very polite," Limon said, "but it was still very disconcerting to him. . . . I asked him if they told him it was optional, and he said, 'No; they didn't.' "

Also yesterday, Health and Human Services Secretary Tommy G. Thompson announced that he is offering state health departments 20 percent of their annual bioterrorism grants immediately. The $14.2 million could be spent on smallpox vaccinations, hospital improvements, training for chemical and nuclear attacks and emergency planning.

The administration also is loading portions of a national emergency medical supply stockpile onto planes and trucks to allow for quicker distribution in an emergency, said Jerome Hauer, acting assistant secretary for public health preparedness at HHS.