No-Fly Zones, High Security for July 4

July 1 -- As Americans prepare to celebrate the Fourth of July, officials are preparing a range of heightened security measures for what could be a tempting target date for terrorists.

"What could be more symbolic than thinking about an attack [on the] American-only holiday of the year, which is the Fourth of July?" asked Jerry Bremer, a security consultant and former chairman of the National Commission on Terrorism.

The FBI has issued another alert to state and local law enforcement agencies, warning them of a possible terrorist attack somewhere in the United States this week.

The warning was issued to law enforcement agencies, not directly to the public, partly because officials have no specific information about a possible terror plot.

No-Fly Zones, Air Patrols Are Back

The warnings have prompted unprecedented security measures, and government officials are considering boosting the national color-coded terror alert from yellow to the more critical orange, law enforcement sources told ABCNEWS.

The Pentagon has ordered jet fighters to resume patrolling the skies above major cities, such as New York and Washington, as they did in the days after Sept. 11. The Federal Aviation Administration has established "no-fly zones" around several national landmarks, including Seattle's Space Needle, Mount Rushmore, the Gateway Arch in St. Louis, and the Statue of Liberty.

In New York, the U.S. Coast Guard will check every boat that comes to the East River to watch the traditional Macy's fireworks display. Some 4,000 police officers will be on duty for the holiday.

The hundreds of thousands of people who are expected to celebrate at the National Mall in Washington will find a two-tier fence and layers of security officers.

"Bags will be inspected. Officers will be present," said Teresa Chambers, chief of the U.S. Park Police, adding that officers would use handheld metal detectors and police dogs.

In Boston, the expected half-million spectators on the Charles River will be joined for the first time ever by the National Guard, which will monitor all boats in the area.