Small Aircraft; High Wing; 1 Eng; Fixed Gear
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 1
Human-Machine Interface; Situational Awareness
Conflict NMAC; Deviation / Discrepancy - Procedural Published Material / Policy
Human Factors; Procedure
Working SFRA-S; security monitoring position. Had 3 aircraft landing VKX. Aircraft X was number 3 for the airport. Made traffic call for aircraft in front of Aircraft X who reported traffic in sight. Also reported VKX in sight. Switched aircraft to advisory frequency. Aircraft Y was in comms with DCA tower southbound; abeam of DCA when I switched Aircraft X to UNICOM. Aircraft X was short final to VKX when CA alert went off. Shortly after; Aircraft X returned to my frequency and reported an NMAC with Aircraft Y who was still in comms with DCA tower. I acknowledged the transmission and returned Aircraft X to advisories. Aircraft Y contacted me about 5 miles later and I asked the pilot if he had received a traffic call from DCA tower for the VKX arrivals. He said he did; and he had the traffic in sight and had been maintaining visual separation.I don't have any recommendations. The aircraft were more than 10 miles apart when I switched Aircraft X to advisories. I wasn't talking to either aircraft at the time of the reported NMAC. Aircraft Y had received traffic advisories and reported that he was maintaining visual separation.
PCT TRACON Controller reported a NMAC from an aircraft they were no longer talking to.