STG Inc., a government contractor in technology and engineering services, said yesterday that it will add more than 700 jobs at its Fairfax headquarters in the next three years. As an incentive for the $15 million investment, Virginia pledged to help train employees.

STG already has about 250 of its 1,700 employees in Fairfax. Most of the new employees will be engineers.

"We plan to be bidding on and winning more jobs," said Phillip J. Foote, STG executive vice president.

STG provides protective services for the State Department, telecom services for the Environmental Protection Agency, and data modeling and simulation for the National Aeronautics and Space Administration.

About 95 percent of the company's revenue comes from government.

Privately owned STG said it has annual revenue of $170 million, up about 40 percent in the last year because of acquisitions.

STG bought International Computers and Telecommunications Inc. in April, followed by PSC Inc. in May and Decision Systems Technologies Inc. last month.

STG officials said they chose to expand in Virginia over possible locations in the District and Maryland.

Fairfax County's location and the abundance of engineers in the area with security clearances for government work played roles in that decision, as did a commitment by the Virginia Department of Business Assistance to provide training.

"We've building a closer relationship with the government in terms of career training," Foote said.