The birthplace of modern American espionage has been hiding — as befits a former nest of spies — in plain sight.

The old headquarters of the Office of Strategic Services is a little cluster of stone and brick buildings clearly visible to anyone walking out of the Kennedy Center or zooming around the E Street Expressway. But few know that the Beaux Arts campus is where Gen. “Wild Bill” Donovan and his band of OSS agents invented the spycraft that helped win World War II, the pistol pencils, fake passports and the propaganda broadcasts that led enemy soldiers to doubt their causes (and sometimes their wives).

After the war, the compound became the first home of OSS’s successor agency, the CIA. And yet even to its neighbors, the spot’s remarkable pedigree as the place that bedeviled Hitler and crafted the Cold War might as well be stamped “top secret.”

“I had no idea about its history,” said Patrick Kennedy, a Foggy Bottom ANC commissioner who lives just three blocks from the site at 2430 E Street that is known variously as Navy Hill and Potomac Hill.

But if obscurity was good for the cloak-and-dagger days, it may be bad in an era when public attention can be the best protection from a bulldozer. It was only after the State Department announced plans to redevelop the site for additional office space that OSS veterans learned no one had ever bothered to gain historic status for their home. Rumors spread that the structures could be demolished, so some aging spooks have come in from the cold of retirement to fight one more mission for the old HQ, joining with local preservationists to save at least some of it for posterity.

Allen W. Dulles, former director of the Central Intelligence Agency, is seen in his Washington office on July 28, 1954. (AP)

“I thought it was outrageous,” former OSS and CIA agent Hugh Montgomery said of the possible loss of the offices that once directed his secret operations behind enemy lines. “I think we all assumed it was already protected.”

The 90-year-old Montgomery, who recently retired from a 63-year career in intelligence, testified at several meetings of the D.C. Preservation League and helped compile information for the league’s application for historic landmark status. Several other surviving OSS vets and their descendants have also contributed memories, documents and photographs to the effort.

“I call them the most dangerous band of 98-year-olds in the country,” said Charles Pinck, the son of a former agent and president of the OSS Society, an alumni group. “It kind of caught us all off guard. We thought, ‘Oh my God, are they going to tear down this historical treasure?’ ”

The General Services Administration, which is directing the building project, says those fears are unfounded. During a recent tour of the site, agency officials said they are committed to protecting the heritage not just of the OSS headquarters but other historic properties on Potomac Hill, including the original Navy Observatory and buildings that housed the precursor to the National Institutes of Health.

“This is a very important place,” said Mina Wright, the GSA’s director of planning and design, as she stood outside the office used by Donovan, the charismatic founder of the OSS. The State Department now uses the space as a conference room, but it is still lined with photos of Donovan and a case displays some of the medals he won as the most decorated officer of the first World War. “We are totally dedicated to protecting the sense of history here, which will depend on the successful integration of the old and the new.”

The agency has begun its own process of getting the compound on the National Register of Historic places and next month will hold a meeting to gather public comments about the setting. It will take more than a year to develop a master plan for the site, Wright said.

“The whole point of doing this is to be very thoughtful about how to approach the whole campus,” she said.

Walter Bedell Smith, left, former director of the Central Intelligence Agency, and aide John S. Earman, pose at the White House on Dec. 1, 1950, as they arrive for President Truman's conference with congressional leaders on Korea. (AP)

But absent a flat guarantee that the facades won’t be lost, OSS supporters want to ensure their HQ won’t be reduced to a bronze plaque.

“They deserve to be saved,” said retired General John Singlaub, 92, who was dispatched from these buildings to missions in occupied France. Later, he was sent to Manchuria as CIA station chief when the young agency was housed in the same offices. “It’s more historically correct to preserve these than some other buildings I see in Washington.”

Not even many history buffs knew much about the heritage of the hard-to-reach compound. The sloping site overlooking the Potomac, once directly above the stately Heurich Brewery, is now bound on one side by the expressway and on the other by the restricted streets surrounding the State Department.

“It’s a mystery why it was never listed [as a historic landmark],” said Peter Sefton, an architectural historian with the D.C. Preservation League. “It’s tempting to think it was a secrecy thing, but it was more likely just lost in the rush of events after the war. I’ve lived in Washington 45 years and didn’t even know it was there.”

Elizabeth “Betty” McIntosh, who is 99 and lives in a Prince William retirement home, still has clear memories of her wartime duty at OSS headquarters, where the days were a mix of stifling clerical routines and thrilling glimpses of intrigue. In a memoir of her OSS days, which provided background material for the historic landmark application, she recalled stumbling into a demonstration of secret gadgets. One was a pencil that shot a single bullet. Another was a chemical that, if splashed on an enemy official in a crowd, would drench him in a foul, fecal odor for days.

“You never knew what was in all those messages being carried back and forth,” said McIntosh, who went on serve as a field operative herself, at one point passing explosives to a saboteur who would blow up a Japanese troop train.

Montgomery said the cables he received in the field were usually just signed “109,” which was the room number of Donovan’s office. “That’s how he would tell us what mischief we were to undertake next,” he said.

The compound was a hive of adventurers in between secret missions, brainy cryptographers and a parade of future leaders. Future CIA directors Allen Dulles and Richard Helms were both OSS alums.

There was star power, too, in the blocky, overcrowded offices. Actor Sterling Hayden was an agent. Donovan enlisted the German-born bombshell Marlene Dietrich to take risky morale-building tours of combat units. McIntosh remembers one of her colleagues taking dictation from Hollywood director John Ford, who commanded an OSS Field Photographic Unit.

One of McIntosh’s regular lunch buddies was Julia Child, an OSS classified file clerk who became on overseas operative before she became a famous chef. The two traveled together to China toward the end of the war.

“It was a fascinating place to work,” said McIntosh. “I think they should save them if they can.”