In 2000, at the pinnacle of the technology boom in the D.C. 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, begging clients to inhabit the windowless fortresses that were built to house large servers that run the high-speed Internet and telecommunications functions.

Developers thought that with the onslaught of Internet use, the demand for secure, specially engineered buildings would skyrocket. During the boom, technology companies were asking for buildings with special specifications: concrete roofs, strong floors, super cooling systems and unbreakable windows -- or, even better, no windows.

But officials in Loudoun County, home to America Online Inc. and companies that no longer exist, such as PSINet, are starting to look at the buildings as if they were 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 economic development department. "There were so many people building data centers."

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

"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 of CB Richard Ellis, a commercial real estate company.

At the same time, Schweiker said, more corporate users are looking for 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.

Three squat, concrete buildings are plunked along the Dulles Greenway. 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 planned an additional 1.4 million square feet of data-center space along the Greenway. But there are no signs that construction 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 turned into regular industrial use, such as a storage facility with loading docks, unless Archon goes through a cumbersome rezoning process.

Several messages left with Archon were not returned.

Archon owns a separate 16.46-acre parcel, also zoned office, and 17.74 acres zoned industrial. 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 more data centers.

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

The county has a 25 percent vacancy rate for industrial/flex space. Empty data-center space is responsible for much of that high rate, Bailey said. There are two other large data centers that remain empty: a 120,000-square-foot data center built by Trammell Crowe and an 80,000-square-foot data center built by BF Saul Co. Both data centers 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 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.

Bailey said she can't regret the decision to allow the data centers to settle in the county. She is holding out hope that the centers will be occupied as the commercial real estate market starts to come back from its major slump.

"There may be a market for them. We haven't given up hope there may be some requirements that come back," she said. "I wish I had my crystal ball."

Data centers, such as this one being built near Ashburn in 2001, were designed to house large Internet servers.