As Americans gather for July Fourth festivities, security officials will be watching for terrorists.

There's no specific threat, and the Department of Homeland Security (search) hasn't raised the national terror alert. But it's urging law enforcement officials to increase their presence at weekend events.

Last week, the agency sent a bulletin urging state and local officials to tighten security at power and chemical plants and key transportation facilities.

In Pennsylvania, the National Guard and state police will be guarding the state's five nuclear power plants around the clock at least through Independence Day. Governor Ed Rendell says he made the move "in an abundance of caution."

In downtown Washington, workers are putting up fencing and setting up checkpoints for the hundreds of thousands expected on the national Mall tomorrow. Officials say all backpacks, bags and coolers will be searched.

Holiday events include a parade, a National Symphony Orchestra concert and, of course, fireworks.

A constant stream of intelligence indicating that Al Qaeda wants to strike the United States this summer or fall has led federal officials to urge increased vigilance during the Independence Day weekend, but there is no specific threat of an attack timed to the holiday.

In a bulletin sent to law enforcement agencies nationwide Thursday, the FBI said police should step up patrols and watch for signs of terrorist activity, including surveillance of potential targets. The FBI also listed advice on how to spot possible suicide bombers.

"We know the U.S. homeland remains a top Al Qaeda target," the FBI said in the bulletin, sent weekly to 18,000 state and local law enforcement agencies.

Fourth of July celebrations (search) are among the symbolic events that U.S. officials say could present an inviting target to Al Qaeda, which intelligence reports indicate will attempt an attack during summer or fall. An attack also could be timed to coincide with the national political conventions or the November elections, plus the Olympics (search) in August in Athens, Greece.

Homeland Security Secretary Tom Ridge said the warnings were not issued because of a specific threat.

"The information that the FBI put out is just a reminder," he said Friday on a morning news show. "There are some lessons we've learned from observing the Al Qaeda and other terrorist organizations conduct their attacks around the world."

The FBI said police should increase patrols this holiday weekend, vary the timing, size and routes of the patrols, and make sure all vehicles illegally parked in key areas are approached and their drivers questioned.

The Homeland Security Department had no plans to raise the nation's color-coded terror alert level above its current midpoint status of yellow, or elevated. Last week, the agency sent a bulletin urging tighter security to state and local officials and those that operate power and chemical plants and key transportation facilities.

The government's approach to this major U.S. holiday was muted in comparison to Memorial Day, when the FBI and Attorney General John Ashcroft issued high-profile warnings that terrorists were nearly ready to strike.

In Florida this week, Ashcroft repeated his contention that Al Qaeda was between 75 percent and 90 percent ready to attack again.

FBI Director Robert Mueller told reporters Thursday in Seattle that local, state and federal officials must cooperate to prevent a new terrorist attack. The lack of such cooperation, from the federal government on down, has been cited by congressional and independent investigators as a key reason the Sept. 11, 2001, plot was not detected.

"We try to use our imagination to determine where the next threat might come from," Mueller said after a visit to the FBI's Seattle field office.

The FBI bulletin cited recent intelligence that continues to show Al Qaeda interest in attacking a range of facilities, including gasoline stations and refineries; financial and government institutions; civil aviation; nuclear plants and dams, and subways and freight trains.

Terrorists could seek to replicate attacks overseas that have used bombs in vehicles, assault teams armed with light weapons and suicide bombers, the FBI said.

Around the country, state and local officials echoed the federal government's approach to the Fourth.

"We're encouraging New Yorkers to enjoy the holiday but to remain vigilant and aware of their surroundings," said Lynn Rasic, spokeswoman for New York Gov. George Pataki. "Security will be strong and noticeable at public events across the state."

In Atlanta, the city transit system canceled days off for police and planned random sweeps of trains with bomb-sniffing dogs, spokeswoman Kimberly Willis Green said. And in Washington, police were setting up 19 security checkpoints along the National Mall for people to enter to watch the nationally televised fireworks show on Sunday.

The FBI bulletin repeated for local authorities a previously released list of indicators often associated with suicide bomber attacks overseas. These include:

— Irregular, loose-fitting clothing not appropriate for warm weather, possibly with "protruding bulges or exposed wires" or a noticeable chemical odor.

— "Sweating, mumbling (prayers) or unusually calm and detached behavior." In addition, people who refuse to show their hands, possibly to conceal a detonator.

— Disguises, including military, police, medical or firefighter uniforms or someone posing as a pregnant woman.

— Large or heavy baggage not appropriate for the location, such as a big duffel bag carried into a restaurant.

Law enforcement officials also were asked to be wary of possible terrorist surveillance, which the FBI said nearly always occurs prior to any attack "to determine suitability, security and probability of success."

Terrorists may also make anonymous threats to observe how security reacts and may attempt surveillance disguised as homeless people, shoe shiners, street vendors or street sweepers, the FBI said.