In 2000, at the pinnacle of the technology boom in the Washington area, large, impenetrable data centers were the hottest thing going in Northern Virginia commercial real estate.

But today, many of those buildings are decorated with "for lease" signs in front, begging clients to inhabit the windowless fortresses that were built to house the large servers that perform high-speed Internet and telecommunications functions.

With the rise of Internet use, developers thought the demand for secure, specially engineered buildings would skyrocket. During the boom, technology firms were asking for buildings with special specifications: concrete roofs, strong floors, super cooling systems, indestructible windows or no windows at all.

In the fall of 2000, a San Jose-based firm called US DataPort announced it would build an $800 million data-center campus at Virginia Gateway in Gainesville. The announcement was lauded by county officials, who were vigorously going after data-center development, believed to be the hot property of the day.

Today, US DataPort is nowhere near Prince William County. The company develops hotels and shopping centers, or, as company President Grant Sedgwick put it, more "conventional real estate."

"We stopped work on that [campus] about two years ago. There was a lack of demand for data-center space," he said. "We were in Northern Virginia purposely because data-center space was strong [there]. We had a great plan but should have brought it forward a year or two earlier."

Officials in Loudoun County, home to America Online and former home to companies that no longer exist, such as PSINet, are starting to look at the buildings as concrete albatrosses.

"I think we all were questioning how deep the data-center market would be," said Robyn Bailey, manager of business infrastructure for Loudoun County's Department of Economic Development. "There were so many people building data centers."

Real estate experts now say that data-center developers overestimated the demand for the buildings.

"A lot of them are either still sitting out there vacant or they're looking at the possibility of how can we convert this to more of a flexible-type use," said Malcolm Schweiker, senior vice president at CB Richard Ellis.

At the same time, he said, more corporate users are looking for some data-center space, but not on the massive scale companies thought they needed in the late 1990s. "Technology has changed so much, [companies] don't need as much space because technology has gotten smaller," he said.

Plunked along the Dulles Greenway are three squat concrete buildings. The Archon Group LP, a Texas-based real estate company, completed the Genisus Loudoun Exchange in 2001. Today, those buildings sit empty. A sign in the grass near the road boasts telecom space for lease. The 432,000-square-foot project is only the first phase. A second phase, which was to be 406,000 square feet, was never started.

In an interview in 2001, Archon officials said they still planned an additional 1.4 million square feet of data-center space along the Greenway. But there are no signs that construction plans will ever come to fruition.

Part of the problem now is that the three empty data centers, which sit on 41 acres, are zoned for office space. That means the one-story buildings, which look like large concrete boxes, cannot be put to regular industrial use, such as a storage facility with loading docks. Unless, that is, Archon goes through a cumbersome rezoning process.

Several messages left with Archon were not returned.

Archon owns a separate 16.46 acres, also zoned for office space, and an additional 17.74 acres zoned for industrial use. But Archon is "definitely not moving forward with data centers," Bailey said.

"Everyone's kind of scratching their heads, wondering what to do with them," said Josh Hinman, a spokesman for Tucon Construction Corp., which built the data centers. The company has not had a call to build any more data centers.

Tucon recently moved onto a different project that is popular on Loudoun's list of construction needs: libraries and schools. The company recently constructed Ashburn Library and is due to finish Smart's Mill Middle School in Leesburg this fall.

The county has a high vacancy rate, 25 percent, for industrial/flex space. A lot of those vacancies are empty data-center space, Bailey said.

Two other large data centers remain empty: a 120,000-square-foot data center built by Trammell Crow Co. and an 80,000-square-foot data center built by B.F. Saul Co. Both sit at Loudoun Tech Center, just east of Routes 7 and 28.

"There are still alternative uses for them," said Allen Tucker, vice president with Staubach Co., a commercial real estate company. "The key is seeing someone who is really going to take a leap of faith and build into a shell building when there are other buildings that exist that are built out."

Tucker compared changing a shell data center into an office building with the idea of changing a brick Colonial house into a wood-structure contemporary.

"It can be done, but it may not be cost-effective to do it," he said.