Al-Qaeda is still planning an attack similar to the September 11th strikes, using hijacked aircraft as missiles, the United States has warned.
The FBI said terrorists - who might disguise themselves as women - may try to poison food or water supplies.
An advisory issued ahead of the second anniversary of the September 11th attacks, says intelligence reports attackers may try to hijack aircraft that fly near, but not into, the United States so they will not have to pass the increasingly stringent requirements to board an America-bound flight.
It also warns of truck bombs at targets such power plants, petrochemical facilities, transport hubs, dams and food distribution centres.
But the FBI has not detected any individuals or group planning attacks such as those almost two years ago that killed some 3,000 people in New York, Washington and Pennsylvania.
The Homeland Security Department adds it has 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.
Moreover, the FBI bulletin said there is no intelligence indicating that al-Qaeda is planning an attack to coincide with the September 11 anniversary.
The FBI's latest weekly bulletin to state and local law enforcement agencies cautions terrorists might use the toxins nicotine and solanine, which is found in potatoes that are old or have been exposed to sunlight for a long time.
The bulletin says terrorist manuals and documents recovered at al-Qaeda sites in Afghanistan contain references to use of both substances as poisons.
AP