Two Prince William County data centers -- fortress-like bunkers that have stood largely vacant for four years as relics of the tech boom -- are back in business and should soon be housing new tenants.

Owners of the two facilities say demand for secure computer housing has increased alongside the resurgence in local tech business and investment. Windowless concrete shells, data centers have special climate controls and backup power supplies.

They have also in recent years been a disappointment for Prince William County, which had high hopes for projects such as an America Online Inc. data center that was supposed to employ 175 people.

But now, "the data-center business is back," said Martin Briley, executive director of the Prince William County Department of Economic Development.

Dulles-based AOL recently sold its 230,000-square-foot building to Porpoise Ventures LLC, which plans to lease the facility this fall to an unidentified tenant. Porpoise is a subsidiary of DuPont Fabros, a District-based commercial and residential development firm which specializes in data centers.

Officials from AOL and DuPont Fabros did not disclose terms of the deal, but county land records indicate the building and the 31.9 acres it sits on sold for $58.5 million on June 28.

In 2000, AOL bought the property at 8217 Linton Hall Rd. for $3.3 million and a year later opened the data center it built. When plans for the facility were announced, it was described as a $555 million investment.

"We never came close to reaching that goal for a variety of different reasons" including changes in the market and AOL's technology and strategy, said Nicholas Graham, an AOL spokesman. "In the end, there was no real net positive or net negative financial impact on the company as a result of this real estate purchase and sale."

AOL officials also planned to employ 175 workers at the facility, and in exchange, Prince William County offered the company $557,500 in economic incentives while the state provided another $600,000. AOL officials said they have until the end of July to meet any financial obligations to the county and state as a result of that benefits package.

"AOL has always and will continue to meet any financial obligations it has to the county and the Commonwealth as part of this real estate transaction," Graham said.

Fortress Development Co., an affiliate of Rockville-based BECO Management Inc., has also leased its data center, known as "Fortress Manassas" or "Cyberfortress," to an unidentified tenant to move in by the end of the year.

The 109,800-square-foot facility was completed in September 2001 and was supposed to be the first of three data centers totaling 250,000 square feet built at the Battlefield Business Park. But no tenants came because of the failure of technology companies. Officials thought they might lease the building to government agencies that needed heightened security after Sept. 11, 2001.

"That really didn't materialize," said Chris Epstein, president of Fortress Development.

Now, companies that need to secure data and equipment are occupying such facilities once again, he said.

"For a number of people, they thought those facilities had come and gone," Epstein said.

Chris Epstein is president of Fortress Development, an affiliate of BECO Management of Rockville that has leased its data center known as "Fortress Manassas" or "Cyberfortress."