The FBI's warning of a possible terrorist attack as early as Tuesday was met largely with calm attentiveness by law enforcement agencies.

The nation's police officers, already on high alert since the Sept. 11 attacks and subsequent federal warnings, wondered how much more cautious they could be.

"Ever since the Sept. 11 terrorist attacks, our officers have been on high alert and we continue to remain so," said Los Angeles Police Department spokesman Jason Lee.

"No specific location or time was given for this latest alert, so there's not much more we can do. I mean how much higher can we go? We're already on the highest alert."

The FBI scrambled to put out the warning on Monday, saying "recent information indicates a planned attack may occur in the United States or against U.S. interests on or around Feb. 12, 2002. One or more operatives may be involved in the attack."

About 18,000 law agencies were notified, but officials were not specific about possible targets. The alert identified one possible attacker as Fawaz Yahya al-Rabeei, a light-skinned 22-year-old from Yemen.

New York Mayor Michael Bloomberg urged people to "go about your business" on Tuesday.

"I don't think you should panic," Bloomberg said.

New York City police already were on "heightened awareness" even before the warning, with officers stationed at landmarks, tunnels, bridges and city, state and federal office buildings.

Part of the city's Port Authority Bus Terminal was temporarily evacuated Tuesday morning after a man allegedly placed a bag in front of a police desk and told a bus company supervisor that it contained a bomb.

The supervisor alerted police, who immediately arrested Eduard Poplawski, 56. He was sent for a psychiatric evaluation and charged with falsely reporting an incident, placing a false bomb and possessing marijuana.

Elsewhere, police said they were remaining calm after the latest advisory. A spokesman for Michigan State Police said his agency was aware of the FBI alert but had no reason to believe the state was particularly likely as a site for an attack.

"It was more or less a 'be on the lookout' alert," said Lt. Jerry Conners in East Lansing.

Before Monday, FBI and Homeland Security officials had issued three general alerts urging all Americans to be cautious and on the lookout for possible terrorist activities.

The last was issued Dec. 3 and was supposed to last through the holidays. It has since been extended through the Winter Olympics in Salt Lake City and is supposed to expire around March 11.

In addition, the FBI has issued numerous narrower alerts, such as to nuclear power plant operators. And some state leaders have added to the urgency, like California Gov. Gray Davis' warnings for state bridges.

In Oregon, law enforcement agencies and officials at Portland International Airport said they had no plans to make any security changes because of the most recent alert.

Officers in Concord, N.H., likely would be doing extra patrols of potential targets -- state buildings, water treatment facilities, malls and airports, said Police Lt. Walter Carroll.

But Carroll said that unless authorities get more detailed information about the threat, "You probably wouldn't notice anything different," he said.

Even Wright-Patterson Air Force Base, in Dayton, Ohio, had not changed its security measures. "But as always we maintain a keen awareness and constant vigilance," said base spokeswoman Sue Murphy.

In Nebraska, Omaha Officer Meg Fricke said the FBI alert was forwarded throughout the department, but nothing in particular was planned.

"Are we locking down public buildings? No," she said. "It's just kind of business as usual."