Visitors to Washington's Fourth of July festivities Sunday can expect to encounter tight security, something that in the past three years has taken its place alongside fireworks and free concerts as a holiday tradition on the Mall.

This year's Independence Day events come five weeks after top U.S. law enforcement officials said al Qaeda might launch a major attack on U.S. soil this summer or fall. In late May, Attorney General John D. Ashcroft and FBI Director Robert S. Mueller III listed several major events -- including Fourth of July celebrations -- as possible terrorist targets.

The Department of Homeland Security, however, chose not to raise its color-coded terror threat level from yellow, or "elevated." Secretary Tom Ridge encouraged Americans to enjoy the summer as they normally would, while keeping an eye open for anything suspicious.

Officials with the U.S. Park Police and other agencies working on Sunday's event say they plan to heed that advice. "We try to strike a balance between ensuring safety and keeping an open atmosphere so the public doesn't feel boxed in," said Sgt. Scott Fear, spokesman for the Park Police.

Visitors to the Mall will enter via 19 security checkpoints, the same number as last year. All coolers, backpacks, packages and people will be subject to inspection, and alcohol, glass bottles, fireworks and personal grills are prohibited.

Fear said that although security will be similar to last year's, some details have changed. For example, he said, some of the search techniques may differ this year, and the locations of road closures and checkpoints may vary.

"We always try to change it up a little just in case somebody was casing it out last year," Fear said.

Officers from more than 20 local, state and federal agencies will join the Park Police on the Mall. FBI officials said they, the area's Joint Terrorism Task Force and the National Capital Area Response Team plan to operate their command center all day Sunday.

"We will have hundreds of officers on foot, on horses, with canine, in cars and at fixed posts," said U.S. Capitol Police Chief Terrance W. Gainer. "Between the Metropolitan Police Department, the Transit Police, the Park Police and the U.S. Capitol Police, this will be the most secure place in America to celebrate."

Capitol Police Deputy Chief James P. Rohan added: "It's just an historically significant event . . . and it's a large-scale event. We're just trying to make sure we have an appropriate security posture for the entire event."

Several hundred D.C. police officers will assist the Park Police at checkpoints, help control crowds and direct traffic. The department's helicopter and harbor boats will be deployed during the celebration to add an extra level of security. And, officials said, the force's 14 surveillance cameras will be activated to help monitor the festivities from the command center at police headquarters. Police officials said they restricted some leave to ensure adequate patrols in city neighborhoods.

"Anytime we have a major event, we just want to make sure nothing happens," said Executive Assistant Chief Michael Fitzgerald.

Metro officials said trains will run from 7 a.m. to midnight Sunday. As in the past two years during the celebration, the Smithsonian Metro station will be closed to allow police to more easily screen Mall visitors. Polly L. Hanson, Metro Transit Police chief, said all of her agency's employees -- from newly sworn-in recruits to bomb-sniffing canine units -- will work Sunday.

"People used to just worry about drunk people and too much noise from the Beach Boys," said Hanson, who has worked all but two Fourth of July celebrations on the Mall since 1975. "But events in the world have changed all that."

The fireworks are scheduled to start at 9:10 p.m., preceded by a full day of events.

The Smithsonian Folklife Festival will continue on the Mall from 11 a.m. to 5:30 p.m. Sunday. A two-hour parade will begin at 11:45 a.m. along Constitution Avenue from Seventh to 17th streets NW. At 5:45 p.m., the National Park Service's Independence Day concert will begin on the Washington Monument grounds, near 17th Street and Independence Avenue SW. A concert featuring the National Symphony Orchestra will begin at 8 p.m. on the U.S. Capitol's West Lawn.

Staff writers Sari Horwitz and Del Quentin Wilber contributed to this report.

This year, as in 2002, above, people attending the Fourth of July celebration on the Mall will go through checkpoints and be subject to search.