One of the four is a Saudi with family ties in South Florida, who has previously been described as a possible al Qaeda operative in the mold of Mohammed Atta, ringleader of the 9-11 attackers.
The FBI posted the bulletin on its Web site and circulated it among law enforcement agencies after recent intelligence indicated the four could be involved in an unspecified plot against U.S. interests, said a law enforcement official who spoke on condition of anonymity.
The FBI had been seeking information about all four — two Saudis, a Moroccan and a Tunisian — for months, but the new information led officials to intensify the search, said a second official, also on condition of anonymity.
"These individuals should be considered armed and dangerous," the FBI bulletin reads. None of the four are believed to be in the United States.
The four men are identified as Adnan G. El Shukrijumah, 28, a Saudi native with ties to South Florida; Karim El Mejjati, 35, a Moroccan who holds a French passport and last entered the United States between 1997 and 1999; Zubayr Al-Rimi, 29, a Saudi; and Abderraouf Jdey, 38, a Tunisian who may have a Canadian passport.
The bureau says it's possible El Mejjati helped plan a suicide attack using explosive vests in Morocco last May, killing 29 people.
But CBS News Correspondent Jim Stewart reports it's Adnan Shukrijumah who worries officials the most. He's a Florida college graduate who the FBI says is being groomed as the next Mohammed Atta, the ringleader of the 9-11 hijackers two years ago.
Shukrijumah disappeared from his Miramar, Fla., home before 9-11 and hasn't talked with his family in more than a year. His name surfaced on paperwork in al Qaeda training camps in Afghanistan, and last March captured terrorist leader Khalid Sheik Muhammed also identified him as a budding new cell leader.
His father denies it.
"I do not believe that. Because I know my son was not a violent person," Gulshair Shurkrijumah says.
The FBI has not detected any individuals or cells actively planning attacks such as those almost two years ago that killed some 3,000 people in New York, Washington and Pennsylvania. Most al Qaeda operatives in the United States provide logistical support such as travel documents, recruitment and fund raising, said Larry Mefford, the FBI's chief counterterrorism official.
Separately, the Homeland Security Department is advising federal, state and local security officials to evaluate their security procedures in the run-up to the anniversary of the Sept. 11 attacks, but said current intelligence doesn't warrant an increase in the national threat warning.
The current code yellow alert, which signifies an elevated risk of terrorist attack, is the middle level on the five-color scale.
The Homeland Security advisory issued on Thursday, says, "We remain concerned about al Qaeda's continued efforts to plan multiple attacks against the U.S. and U.S. interests overseas." It adds that they have no specific information on individual targets or dates for any attack.
Recent arrests of senior al Qaeda members appear to have slowed some of their operational planning, the advisory says.
However, it notes that al Qaeda operatives are still looking to conduct an attack similar to the Sept. 11 strikes, using hijacked aircraft as missiles.
The Homeland Security advisory says intelligence indicates operatives may try to hijack flights that fly near, but not into, the United States, so they will not have to pass the increasingly stringent requirements to board a U.S.-bound flight.
It also warns of truck bombs at infrastructure targets, like power plants, petrochemical facilities, transportation hubs, dams and food distribution centers. Lightly protected targets like restaurants, hotels and apartments are also possible targets, it says.
Terrorists could also try unusual approaches to artfully conceal suicide devices, the advisory said. "Male bombers may dress as females in order to discourage scrutiny."
The FBI's latest weekly bulletin to state and local law enforcement agencies cautions terrorists might use two naturally occurring toxins – nicotine and solanine – to poison U.S. food or water supplies. Nicotine is found in tobacco plants and solanine in potatoes that are old or have been exposed to sunlight for a long time.
The bulletin, obtained Thursday by The Associated Press, says that terrorist manuals and documents recovered at al Qaeda sites in Afghanistan contain references to use of both substances as poisons.
The FBI said there are no known uses of either toxin by al Qaeda or other Islamic extremist groups, and there is no intelligence indicating such an attack is imminent. But the bulletin noted a Michigan man pleaded guilty in May to lacing 250 pounds of ground beef with an insecticide containing nicotine, sickening 92 people, in an attempt to get a supermarket co-worker in trouble.