German authorities are seeking two fugitives believed to have helped plot the four deadly hijackings that killed thousands last week in the United States.
The FBI, meantime, sent out precautionary warnings to law enforcement organizations and utilities to increase security and be on guard for possible attacks.
And several Hollywood studios halted tours and increased armed patrols after a general warning from the FBI that television and movie facilities could be targets of terrorist attacks.
German prosecutors issued arrest warrants for Ramzi Binalshibh, 29, a Yemeni national, and Said Bahaji, 26, a German of Moroccan origin.
Both are suspected of helping plot the Sept. 11 attacks on New York and Washington with three of the hijackers who lived in Germany, prosecutors said. They are being sought on charges of forming a terrorist organization and at least 5,000 counts of murder.
Both Binalshibh and Bahaji lived in Hamburg, a city under intense scrutiny by investigators because three of the 19 suspected hijackers had lived and studied there.
Bahaji lived in the same apartment as Mohamed Atta, identified by the FBI as a hijacker of one of the planes that hit the World Trade Center. Binalshibh was registered as living at the same address.
Bahaji left Germany early this month, flying via Turkey to Pakistan; and Binalshibh was last seen in Hamburg in August, prosecutors said.
For months before last week's attacks, Osama bin Laden and his far-flung forces kept Washington off guard by using decoy terrorist teams overseas and feeding disinformation over phone lines and other communications systems monitored by U.S. intelligence, officials said Thursday.
This year, the United States has shuttered scores of embassies and issued repeated warnings to Americans traveling or working abroad, citing credible terrorist threats in Africa, the Middle East, the Far East and elsewhere.
In the most dramatic case, the Pentagon in June abruptly halted military exercises in Jordan, ordered U.S. 5th Fleet warships in Bahrain out to sea and called the highest state of alert for U.S. military forces in the Persian Gulf after intercepted communications indicated an imminent attack by bin Laden terrorists. Nothing happened.
A U.S. intelligence official told the Los Angeles Times that investigators now suspect some recent threats were deliberately designed to keep Washington off balance before last week's attack.
Another U.S. official said bin Laden used fake threats, and even decoy terrorist teams, in the past year to "probe our defenses" and see how the U.S. government would respond.
In another development, the Washington Post reported today that a series of false bomb threats were made against airliners at the same time that hijacked planes were flying toward New York and Washington last week.
Government investigators told the Post they believe the bomb threats were the work of accomplices of the hijackers attempting to confuse air traffic controllers.
Investigators began to suspect a possible link between the hijacking and the false bomb threats when they interviewed an air traffic controller in the Cleveland control center who had charge of United Airlines Flight 93 on Sept. 11 when it turned south toward Washington, ultimately crashing in a Pennsylvania field.
Concerned that it had taken about 15 minutes for the controller to report Flight 93 as a possible hijack, investigators discovered that he was attempting to get at least two planes safely to an airport after they had received bomb threats.
Investigators also discovered that similar threats had been made in the airspace of the Boston control center, which oversees New England and part of New York.
In other developments in the investigation into last week's attacks:
The FBI asked the nation's water companies to increase security at their facilities.
The Federal Aviation Administration issued a notice prohibiting flights in the immediate vicinity of any major professional or collegiate sporting event.
In Pennsylvania, at the site of the Sept. 11 crash of United Airlines Flight 93, FBI Director Robert S. Mueller III said the FBI has doubts about the identities of several of the 19 hijackers whose names were on passenger lists and had been released by the FBI last week. Saudi newspapers have reported that some of the men are alive; some were pilots.