(AP) Officials say the FBI is putting together a major national operation to monitor and protect July Fourth parades and festivities because of concern that terrorists might attack.

Officials said the agency is requiring its 56 field offices to create specific plans for monitoring events in their respective regions.

The plans are due at FBI headquarters this week.

One source says the plans aren't based on any specific threat.

But terrorist detainees at Guantanamo Bay Naval Base in Cuba have revealed a "general interest" in striking on that day.

The monitoring will probably include several forms of electronic surveillance as well as a heavy presence of field agents, both apparent and undercover, the officials said.

Parades and festivities in downtown areas near subways or other mass-transit systems are of special concern, the officials said. Those activities would be more susceptible to a biological or chemical attack because of the large number of people packed into a small area.

The government has not put out a public warning or advisory for Americans to be more cautious specifically on July Fourth.

But the Bush administration's Homeland Security office has advised government personnel at many levels that security staffing should remain at normal levels.

"It's important not to reduce the number of guards simply because it's a national holiday," said spokesman Gordon Johndroe. "All sections of government must maintain vigilance. For the government, July Fourth will be just another day at the office."

Cheryl Mimura, a spokeswoman for the FBI field office in Los Angeles, said staffing has not yet been decided for specific events.

FBI and Justice Department officials have said they believe there are al Qaeda operatives awaiting a chance to conduct more terrorist attacks.

Dozens of public and private precautions have been taken by the government over the past several months. Some have been linked to perceived threats against certain targets - the Orlando, Fla., water supply, the Brooklyn Bridge, the Liberty Bell.

Officials believe terrorists have made a point of striking near holidays before.

Robert Reid, 28, a British citizen who authorities allege was trained in Afghanistan by the al Qaeda terrorist group, is accused of trying to blow up American Airlines Flight 63 during its trip from Paris to Miami just before Christmas.

Reid was overpowered by passengers and crew. Authorities said his shoes contained plastic explosives.

The FBI also heightened security last Thanksgiving, closely watching parades in New York and Washington.