It is a source of Baltimore pride, an icon of American history and perhaps the most vivid symbol of Independence Day aside from the flag itself.

But for now, the oldest manuscript of Francis Scott Key's "Star-Spangled Banner," the celebrated poem that became the words of America's national anthem, is off-limits.

And so are many of the other prominent symbols of history that patriotic Americans might be hoping to visit to celebrate the birth of their nation.

Heightened security concerns in the wake of the Sept. 11 terrorist attacks have coincided this year with a recent spate of historical preservation and restoration projects to close off access to a number of historical treasures, including the Constitution, the Bill of Rights, the Declaration of Independence and the Statue of Liberty.

With the approach of Independence Day, the security concerns also have prompted the FBI to warn local police in Washington, D.C., about the possibility of a terrorist strike timed to coincide with the holiday. No specific threat exists, the FBI says, but it also will monitor July Fourth celebrations nationwide.

Yesterday, President Bush urged Americans to "celebrate heartily, because we have freedom and we love freedom," saying the government is doing what it can to thwart future attacks.

For the Maryland Historical Society, which since 1953 has displayed Key's original manuscript in an archival case on the second floor of its West Monument Street building, Sept. 11 was merely the last straw after a string of threats to the document that convinced officials to lock the treasure away.

"On and off, the police had tapped into suggestions that people could kidnap the 'Star-Spangled Banner,'" Maryland Historical Society Director Dennis Fiori said. "When [Sept. 11] happened, it was just sort of the last piece."

A Maryland lawyer, Key penned the poem in 1814 and gave the handwritten manuscript to his brother-in-law, Judge Joseph Hopper Nicholson, for publication.

Nicholson's son, James Macon Nicholson, inherited the document in 1817, passing it on in 1875 to his daughter, Rebecca Lloyd Shippen.

Shippen sold the manuscript in 1907 to Henry Walters, the founder of the Walters Art Gallery, which sold it to the Historical Society in 1953.

The society took down the encased manuscript, which was slated for preservation and restoration work during 2002 anyway, in September 2001 and is scheduled to bring it out again in November 2003.

"We were nervous about having such an icon of American freedom sitting here," said Abby Lattes, associate director of public relations and marketing for the historical society.

A "Star-Spangled Banner" manuscript replica hangs where the original once was. The real thing has been moved elsewhere - historical society officials will not say exactly where - for an elaborate restoration project.

Conservators, who started work on the manuscript in June, will repair creases in the paper and tears on its corners, then examine the document with a fadometer, which gauges a material's resistance to fading.

When the restoration and evaluation are completed, the "Star-Spangled Banner" is to be encased in a new apparatus being made by a boat and scuba dive equipment manufacturing firm, Oceaneering International Inc., of Upper Marlboro.

The case will be filled with argon gas, to protect the manuscript from deterioration due to climate changes.

But even once the manuscript resurfaces, Key's famous poem - once known as the "Ode to Fort McHenry" and inspired by the flag that flew over the fort during the siege of 1814 - will not be as accessible as it once was.

"There's an internal debate about how much time it should actually be on view," said Fiori, citing preservation considerations - not security - as the guiding concern.

Some conservators say preservation standards call for the document to be kept stored and taken out only for special events and during certain prescribed times.

Fiori's goal "is to have it on view as much as possible," he says.

"If they can put the Constitution and the Declaration of Independence on view all the time, I think we can put the 'Star-Spangled Banner' on view."

Funny he should use those examples.

The National Archives and Records Administration put away the documents known as the Charters of Freedom - the Declaration of Independence, the Constitution and the Bill of Rights - on July 5, 2001, and will not be displaying them again until 2003.

"We took them off display not because of September 11 but because they needed to be restored," National Archives spokesman Susan Cooper said.

Like the "Star-Spangled Banner" manuscript, the documents were first placed in archival glass in a helium-filled, airtight container in the early 1950s, and conservators are now in the process of restoring and re-encasing them while renovation on the Archives rotunda and exhibition spaces is completed.

As part of that overhaul, the Archives is constructing a new vault for the Charters - the current one was built in the 1940s - designed to protect them against modern threats.

"The Charters will be back on display," Cooper said. "We feel we have an obligation to do that because these documents belong to the American people, and we need to make sure that they have access to them."

As for the Statue of Liberty, one of the more visible symbols of American freedom, the 151-foot, 1-inch statue remains the only portion of any national park still closed for post-Sept. 11 security concerns.

An eventual reopening is "something we're working on feverishly - we've been working on this nonstop," said Brian Feeney, a spokesman for the National Park Service, which oversees Lady Liberty because she is part of a national monument.

Liberty Island, where the statue sits, and Ellis Island both are open, so visitors still can get near the Statue of Liberty; they just can't enter or make the climb to her crown.

Those determined to see a piece of American history up close still can do so in Philadelphia, where the real Liberty Bell is on display, albeit protected by barricades and flanked by magnetometers - measures added after the Sept. 11 attacks.

"There are obvious signs of security when you come to the park," says Frank Eidmann, the special events coordinator for Independence National Historical Park, which also includes Independence Hall, where the Declaration of Independence was issued, and Congress Hall, where Congress met from 1790 to 1800.

"The consensus of opinion, I think, for most visitors is that we appreciate the fact that you're using appropriate security measures to keep us and our historical treasures safe," Eidmann said.

Closer to home, there are still some historical symbols open to the public.

The Fort McHenry National Monument and Historic Shrine remains accessible, boasting a replica of the flag that inspired the "Star-Spangled Banner," currently being restored and on display at the Smithsonian National Museum of American History.

The Star-Spangled Banner Flag House also is open for business, giving tours of the home of Mary Pickersgill, who made the famous flag that flew during the siege at Fort McHenry and inspired Key's ode.

When the real "Star-Spangled Banner" manuscript goes back on display at the Maryland Historical Society, it will be as part of a Maryland history exhibit planned as the main attraction in the historical society's new gallery, which is now under construction.

"We're planning to use it and the War of 1812 and the siege at Fort McHenry as a link to a discussion of self-security and self-preservation, the liberty to protect yourself," said Laurie Ossman, the curator of the exhibit, titled "Looking for Liberty in Maryland."

"People are very concerned about how to deal with 9/11," Ossman said. "I think it will be in the forefront of people's minds when they think about liberty."

The Associated Press contributed to this article.