From the NWS:
A tornadic thunderstorm moved into eastern Loudoun County from western Fairfax County in the vicinity of the Washington Dulles International Airport. This tornado passed within one half mile of the National Weather Service forecast office in Sterling. This prompted the weather forecast office staff on duty to seek shelter in the safe room constructed in the office. The tornado traveled north from Dulles Airport... just west of Route 28 into portions of Ashburn. The tornado produced some damage on the America online Campus off of Waxpool Rd and more extensive damage to the north in the beaumeade corporate park. Many trees were snapped and uprooted along the path of the tornado in the corporate park. Additionally... three roofs were blown off of buildings and one wall collapsed on one building. The tornado also tumbled two automobiles into the side of a building and turned over a tractor trailer. Based on the damage produced in the corporate park... the tornado reached a maximum intensity of F2 on the fujita scale.