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.