Flight Crew Air Transport Pilot (ATP)
Distraction; Situational Awareness
Conflict Airborne Conflict
Flight Crew Took Evasive Action; Flight Crew Requested ATC Assistance / Clarification
Environment - Non Weather Related; Procedure
[I] entered downwind making all the normal callouts for Runway 35 with one other single engine aircraft reporting in the pattern. That aircraft landed as I began my descending turn to base. As we were turning to base we got a TA alert followed by an RA climb. I commenced a wings level climb to comply with the RA. The Captain spotted the unannounced aircraft closing on us from below and to the left. The distance on the TCAS indicated 200 FT at the closest readout. As the RA cleared and I leveled off at pattern altitude I looked to our left and also spotted the aircraft as it made a left circle and then departed the pattern to the southwest very quickly. After landing; the Captain closed the flight plan with Potomac Approach and inquired about the other aircraft causing a TA/RA and asked if the aircraft might have been a UAV given our proximity to Washington. A Supervisor came on line to answer questions and only acknowledge that they were tracking via radar a VFR target but didn't say where it was. The aircraft we saw was gray in color with a twin boom and a long wing. Different than any normal GA light aircraft I have ever seen. I believe it was a UAV. If; in fact; this was a UAV then the obvious solution is to keep UAVs out of civilian airspace.
A Corporate Jet took evasive action from a TCAS RA target which appeared to be a UAV in the JYO CTAF landing pattern; but PCT had no information about the aircraft when questioned.