WASHINGTON » Hundreds of historic U.S. post offices nationwide face uncertain futures as the U.S. Postal Service downsizes, so preservationists on Wednesday added these American institutions to the list of the country’s most endangered historic places.

Post offices will join the list of America’s 11 Most Endangered Historic Places as a group for the first time. The National Trust for Historic Preservation is citing the bureaucratic process for disposing of thousands of post offices, saying developers and community groups interested in rehabilitating the historic buildings end up walking away when they don’t get timely or clear answers from the Postal Service.

The group also said New York’s Ellis Island hospital complex is threatened, even though it’s a popular historic destination, because the facility where thousands of immigrants received medical treatment upon their arrival has been left open to the elements.

Princeton Battlefield, the site of a pivotal American Revolution episode in New Jersey, also is facing imminent danger from housing development that would change the landscape, preservationists said.

This is the 25th anniversary of the listing of endangered places. Over that time 242 historic sites have been added to the listing. Only 10 sites of those have been lost, while others are still endangered, officials said.

The nation’s post offices represent the largest number of sites that could be lost in towns and cities both large and small. Preservationists began getting calls more than a year ago about individual post offices, so they want to work with the Postal Service to help foster a process for adapting and reusing the historic buildings, said Stephanie Meeks, president of the National Trust for Historic Preservation.

"This isn’t about taking on the post office," she said. "Of course we don’t quibble with the post office having to do what they have to do to manage their business, but we do want to make sure there’s a thoughtful process in place for managing the historic resources."

One developer in Geneva, Ill., ended negotiations with the Postal Service after months of work, citing a lack of clear answers from the agency.

The Postal Service said Wednesday its plans have changed for many post offices since a study last summer. As of last month the agency plans to consolidate about 460 mail processing centers in phases.

Of more than 31,500 post offices nationwide, only 55 are officially listed on the National Register of Historic Places, agency spokes­woman Sue Brennan said. If the Postal Service tries to sell any historic property, Brennan said the agency follows State Historic Preservation Office guidelines to identify historic elements that must be saved.

Another large group of sites being added to the endangered list includes the courthouses of Texas, with support from former first lady Laura Bush. The state’s courthouses were first listed in 1998, but at least 70 of them still need critical repairs. Most are still in use.

Other sites are facing even more imminent threats.

President Theodore Roosevelt’s Elkhorn Ranch in North Dakota’s Badlands, which inspired his views on conservation, is facing development of a road and bridge proj­ect that would "mar" the landscape and "stain Roosevelt’s legacy of conservation," the group said.

Three sites from black history also are being added to the list: Joe Frazier’s gym in Philadelphia where he trained to take on Muhammad Ali, the boyhood home of Malcolm X in Boston and Atlanta’s Sweet Auburn Historic District, where Martin Luther King Jr. was born and later preached.

Sweet Auburn was first listed in 1992 when the area around King’s birth home was at risk. Since then much of the area has been revitalized, but much of the commercial corridor remains vacant and could be wiped out by new development, said Mta­ma­nika Youngblood, president of the neighborhood’s Sustainable Neighborhood Develop- ment Strategies Inc.