One Fairfax County police detective has been killed and at least two other officers have been shot at the Sully District police station after two gunman stormed the building from some nearby woods, police sources said. A civilian police dispatcher also was wounded.

One of the suspects was killed in the shootout with police and the other remains on the loose, the sources said. The two men came from the woods carrying automatic or semiautomatic weapons shortly before 4 p.m. Police do not know their motives.

Fairfax County Board of Supervisors Chairman Gerald E. Connolly identified the slain officer.

"What's maddening is that we don't have a clue who did this," Connolly said. "We don't have motive. Why did they do it? We don't know."

Mike Campbell, the principal of Westfield High in Chantilly, told Channel 4 News that the school is "in lock down" with about 200 students still in the facility and staying in the auditorium. Students will not be allowed to leave the school, which is about a mile from the shooting scene, without a parent or guardian.

Campbell said parents can call the school's main number, 703-488-6300, for more information.