Counterterrorism officials say the threat of Al Qaeda terrorism against the United States remains high even though there are few specifics.
A senior law enforcement official, speaking Tuesday on condition of anonymity, said the amount of intelligence about potential attacks on the United States and its allies has increased this month, rising again to the level before the Sept. 11 attacks.
Gordon Johndroe, the Homeland Security spokesman, said Tuesday that recent warnings about attacks against railways, oil and gas and other economic targets remain in effect.
"There is intelligence, while it is general, that has pointed to and raised concerns about our critical infrastructure," he said.
It is not specific enough to raise the nationwide alert level, which remains at code yellow — the middle of five threat levels — because of a lack of specific details on where and when an attack may occur, Johndroe said. Yellow means "significant risk of terrorist attacks."
Last month's public warnings — issued Oct. 9 and Oct. 23 — came in part from the release of taped statements from high-level Al Qaeda leaders threatening new attacks. Counterterrorism officials regard them as calls to action for field operatives.
The latest tape, with a voice claiming to be Usama bin Laden, was aired Tuesday on al-Jazeera. If validated, the tape would be regarded as significant because it made reference to recent events, the first hard evidence in a year that bin Laden was alive.
In addition, U.S. law enforcement officials said the talk that intelligence is picking up surrounding next week's NATO summit in Prague is similar to what they heard before the July 2001 G-8 summit in Genoa, Italy.
As they did in Genoa, where the gathering of world leaders passed without terrorist incident, security officials are considering closing the Prague city center to all but summit personnel.
Some recent warnings in Europe stem from the same intelligence that led to the U.S. warnings, Bush administration officials said.
Security was increased at a number of European ferry ports and at NATO headquarters in Brussels, Belgium. Counterterrorism authorities also remained concerned about Al Qaeda strikes in the Middle East.
In the United States, one analytical concern is that terrorists will strike to mark the scheduled Thursday U.S. execution of Aimal Kasi, a Pakistani man convicted of killing two CIA employees outside agency headquarters in Virginia in 1993.
Prosecutors contended Kasi was avenging the 1991 bombing of Iraq. He has not been linked to any terrorist organization.
The State Department said last week: "The potential exists for retaliatory acts against U.S. or other foreign interests in response to the execution." Americans around the world should "increase their security awareness" when they visit places where fellow countrymen congregate, such as residential areas, clubs, places of worship, schools and hotels.
Also last week, the FBI issued a bulletin warning of possible attacks being carried out during the holy month of Ramadan, which started Wednesday and runs through Dec. 5.
CIA Director George J. Tenet recently told a congressional panel, "You must make the assumption that Al Qaea is in an execution phase and intends to strike us both here and overseas." He noted recent attacks in Kuwait, Indonesia and off Yemen.