U.S. Homeland Security Director Tom Ridge on Monday warned of more terrorist attacks and asked Americans to return to a high alert.
"Obviously, the further we're removed from September 11, the natural tendency is to let down our guard. We cannot do that. We are a nation at war," Ridge said.
Ridge, without citing a specific threat, said that according to the information collected by U.S. intelligence, there is an increasing possibility of more terrorist attacks.
"The quantity and level of threats are above the norm," he said. "We once again should put the public on general alert."
Ridge urged Americans to view the latest alert as "a signal to be vigilant" and asked them to report suspicious behavior to local authorities.
U.S. federal officials have said the alert should continue at least through the end of the Islamic holy month of Ramadan in mid- December.
The Federal Bureau of Investigation (FBI) has issued the latest warning to law enforcement nationwide as well as to the general public. It was the third warning since the September 11 terrorist attacks and the anthrax letters in October. The FBI issued the two others on October 11 and October 29.