An informant whose tip prompted the FBI to warn of an impending terror attack may have been speaking of the subsequent anthrax attacks that have kept the nation on edge, the FBI director says.

Director Robert Mueller told the U.S. Conference of Mayors he has no hard evidence linking the Oct. 11 warning to anthrax. Still, he raised the possibility Wednesday while defending the alert, a sudden decision that was second-guessed by some local officials because of its vagueness.

Attorney General John Ashcroft was to address the mayors' conference Thursday.

In Alexandria, Va., authorities charged an illegal alien from El Salvador with helping one of the 19 hijackers fraudulently obtain a Virginia identification card. Victor M. Lopez-Flores was scheduled for an initial court appearance Thursday.

Lopez-Flores, accused of helping hijacker Ahmed Alghamdi, is the fourth person charged with aiding the hijackers in obtaining false Virginia identification.

As the nation struggled to keep its mail system safe from anthrax, Mueller also warned the government would respond severely to hoaxes.

That message was felt in Kentucky, were two college students were arrested for a hoax that halted postal service in the town of Murray after white powder spilled from an envelope. Preliminary tests indicated the substance was powdered sugar, Murray Postmaster Mark Kennedy said Wednesday.

Amy Wood, 22, of Benton, Ky., and Erin Creighton, 21, of Morganfield, Ky., both students at Murray State University, were arrested Tuesday. They intended to send the letter to friends, Murray Police Capt. Eddie Rollins said.

The FBI's Oct. 11 warning said the bureau had received information of the possibility of additional terror attacks against Americans inside the United States or abroad in the next several days. The bureau said its information did not identify specific targets, but it asked local police to be on the highest alert and for all Americans to be wary of suspicious activity.

"It is conceivable, although there is no evidence necessarily to support it, that the advent of the anthrax attacks is what this source was talking about," Mueller said in response to a comment by a participant at the mayors' conference. "I must emphasize, there is no evidence ... that the anthrax attacks were a result of organized terrorism."

Mueller was told the mayors heard strong reactions from constituents after the alert, because schools and emergency officials were unsure how to react to the nonspecific threat. The director said the FBI could have done a better job of providing advance notice to local bureau directors, who could then have contacted state and local officials before the alert was made public.

"Whether or not we made the right decision in putting out the alert, I don't know if we would ever know," Mueller said.

"At this point, it is not clear if the few confirmed anthrax exposures were motivated by organized terrorism," Mueller said. "But these attacks were clearly meant to terrorize a country already on the edge. We're responding swiftly to each and every incident."

Dr. Jeffrey Koplan, head of the Centers for Disease Control and Prevention, said in a television interview that despite studies by federal labs, he was unable to tell if the anthrax spores found at various sites in the United States came from the same place.

Dr. Stephen Ostroff, an epidemiologist with the CDC, said the anthrax from the Daschle letter had "exceptionally good dispersible capability," indicating whoever sent it had a scientific background.

"You have to assume whoever is doing this knew what they were doing," he said Thursday on NBC's "Today."

While U.S. officials were dealing largely with anthrax, Pakistani officials said they may have made a breakthrough in tracing movements of individuals related to the Sept. 11 airline hijackings.

Three Western nationals of Arab origin, including one linked to hijacker Mohamed Atta, came to Pakistan shortly before the attacks and may have slipped into Afghanistan, the security officials said Wednesday.

The three were identified by the officials, who spoke on condition of anonymity, as Said Bahaji, a German-Moroccan sought by Germany on an international arrest warrant; Abdullah Hussainy, a Belgian of Algerian origin; and Ammar Moula, a French citizen.

Ashcroft said Bahaji had extensive connections to Atta and fellow hijacker Marwan Al-Shehhi, who were on the flights that crashed into the World Trade Center's Twin Towers.