Science Applications International Corp. doubled its employee count in Harford County -- home of Maryland's Aberdeen Proving Ground -- with its purchase yesterday of a Massachusetts-based firm that specializes in the detection of chemical and biological weapons.

The deal, which was announced earlier and closed for an undisclosed sum, expands SAIC's foothold in a county that gained about 5,000 mostly high-tech jobs during the most recent round of military base closings and realignments.

Through its purchase of Geo-Centers Inc., SAIC gains about 400 jobs in Harford County, adding to its existing base of 400 employees there and making it the county's largest private employer of technology-based jobs, said Gary Boyd, a senior vice president at SAIC.

As a result, the company hopes it will be well-positioned for contract work at the expanding the Army base, said Rep. C.A. "Dutch" Ruppersberger (D-Md.), whose district includes it and surrounding areas.

"When you have a facility like Aberdeen, the partnerships with the commercial sector are so important," Ruppersberger said. "The way for companies to get business [with Aberdeen] is to be right there. SAIC has made a good decision to be close to Aberdeen, and they have a lot of work there now."

SAIC, based in San Diego, has a large federal contracting business based in McLean and 16,000 employees in the Washington area. The company already had three buildings in Harford County totaling 200,000 square feet, Boyd said.

Some of that space accommodates employees that SAIC hired when it was awarded a $390 million contract by the Defense Department in May 2004, Boyd said. Under that contract, SAIC is leading a team of firms that will work to protect 200 military installations against chemical, biological, radiological and nuclear weapons.

As part of that contract, SAIC created a facility in Abingdon that showcases the wares of some 300 manufacturers. Yesterday, the company held an open house there to unveil a new unit, which was created with the acquisition of Geo-Centers, to provide expertise on homeland defense against weapons of mass destruction. The center supports about 200 engineering jobs, Boyd said.