The FBI Monday evening said it was reissuing its warning for Americans to be on the highest alert for terrorism this week in light of new tips the bureau has received pointing to more possible attacks.
The warning, the second such alert this month, was based on intelligence reports that Usama bin Laden's Al Qaeda terrorist network may strike, officials said.
"There may be additional terrorist attacks within the United States or against U.S. interests overseas over the next week," said Attorney General John Ashcroft.
The Justice Department received information about the possibility of an attack on Monday, spokeswoman Mindy Tucker said.
The new warning came as jumpy Americans prepared to celebrate Halloween on Wednesday. Stories circulating on the Internet warned about possible attacks on the children's holiday. Tucker said authorities had received no specific information about a Halloween attack.
Much like the last alert that came Oct. 11, Ashcroft said the source of the information was "credible" but didn't get specific as to the target or nature of the possible attacks.
"We believe this threat to be credible and for that reason should be taken seriously," said Ashcroft.
Monday's alert was almost identical to the first. The latest warning was based on new information the FBI says it has collected since the last high-alert announcement in October.
Officials said new intelligence gathered by investigators indicated that Al Qaeda may be agitating to strike again in the aftermath of the U.S.-led bombings on Afghanistan.
"There certainly is intelligence that causes you to be concerned, and possibly that Al Qaeda may be behind it," one senior U.S. official, insisting on anonymity, told The Associated Press.
Ashcroft advised the public against panicking, but said Americans should be cautious, diligent about reporting any suspicious activities, and patient with tightened security and any new law enforcement measures that might be implemented.
"We ask for patience and cooperation from the American people," the attorney general said.
Ashcroft said 18,000 law enforcement agencies were advised to go on the highest alert. Federal agencies, meanwhile, were increasing security, and immigration authorities were boosting their efforts to keep suspected terrorists from coming onto U.S. soil.
FBI Director Robert Mueller appeared with Ashcroft during the brief news conference Monday evening. He said that local communities are particularly important in preventing further attacks.
Earlier Monday, when asked whether the government expected more attacks from groups associated with bin Laden, President Bush said, "We believe the country must stay on alert, that our enemies still hate us."
Bin Laden is suspected of orchestrating the Sept. 11 suicide attacks on the World Trade Center in New York and the Pentagon in Washington.
While he acknowledged the delicate line officials have to walk in warning the public but not inciting panic, Bush encouraged people to continue their usual activities.
"The American public must go about their lives. I understand it's a fine balance," Bush said.
After the alert went out, Vice President Dick Cheney was spirited Monday night to an undisclosed secure location and remained there Tuesday in order to safeguard the continuity of government in the event of an attack on Bush.
The first of about a dozen outbreaks of the anthrax bacterium — spread through the mail — began appearing about the time of the first alert this month. Three people have died from the illness and others have been sickened with both the milder, skin form and the most serious inhalation strain.
The anthrax found in a letter to Senate Majority Leader Tom Daschle included silica, a crystal commonly used as a drying agent to control clumping in pharmaceuticals. Experts said the presence of silica suggests that whoever sent the anthrax wanted it to float in the air so people would inhale it.
The Associated Press contributed to this report.