Washington Post Staff Writer

Friday, November 26, 2004; Page B01

Shortly after dawn on a recent morning, two dump trucks and a water tanker pulled up to a new, unmarked complex of buildings at East Potomac Park, the grassy peninsula near the Jefferson Memorial. The drivers exited their cabs, knocked at a gatehouse with blacked-out windows and waited for a security guard to emerge from behind a locked door.

A few minutes later, a panel of 10-foot-high security fence slid open, and the trucks disappeared inside, leaving the joggers and cyclists along the waterfront none the wiser about their mission.

What goes on beyond the fence is a mystery. The multi-agency review normally required to erect anything on federal parkland did not apply to the beige, metal buildings. The Navy, which operates the site at Ohio and Buckeye drives SW, calls the work a "utility assessment and upgrade" and volunteers nothing more.

"As a matter of policy, we can't go into the particulars," said a Navy spokesman, Lt. Cmdr. Joseph A. Surette.

Frederick J. Lindstrom, acting secretary of the U.S. Commission of Fine Arts, said it is illegal for him to discuss the matter. He did say the Navy started the work without seeking review from the commission, which oversees the city's Potomac River parklands.

"Let's just say when they're finished, you'll be glad they've done what they've done," Lindstrom said.

Amid the secrecy, theories abound about the four-acre complex, which is dead center in a ring that includes the White House, the U.S. Capitol, the Pentagon, Reagan National Airport and the National War College. Is it a sophisticated sensor station, guarding the 14th Street bridge and other Potomac River crossings? Is it an excavation point for underwater barriers to protect the Washington Channel and Potomac River from submarines? Is it a staging area for Navy Seabees securing underwater cables between the White House and the Pentagon, across the river?

Whatever the case, in a capital where concrete barriers and police roadblocks have become a common and often grating part of city life since Sept. 11, 2001, the Navy compound represents a more ambiguous side to security. It is visible and obscure, hidden in plain sight, billed as temporary but expected to last for years.

And it joins a network of things large and small erected to protect the capital. Wind and radiation tracking instruments sit atop the Federal Reserve. Biowarfare sensors sniff the air in front of the Smithsonian Institution. Antiaircraft systems have been spotted on a rooftop next to the White House and on a Prince George's County riverbank, across the Potomac from Mount Vernon.

For its part, the Navy began work unobtrusively about a year ago on its site near Hains Point, a recreation area known best for its golf course, fishing and "The Awakening," J. Seward Johnson Jr.'s giant sculpture.

The Navy took over the National Park Service land without any announcement. When the agencies that oversee the Mall caught up with the project months later, the hangar-like structures, which cover an excavation area, were visible from Interstate 395.

The agencies' only recourse was to ask the Justice Department to sue the Pentagon. Lindstrom said that was deemed not an option.

After inquiries, senior officials at the Fine Arts Commission and the National Capital Planning Commission were briefed about the security-sensitive project and sworn to silence. The staffs of both commissions, which review many federal agency security requests, were bypassed, and no paper trail was produced.