ACR DSNDING ON APCH HAS TCASII RA. RESPONDS.

Narrative

I WAS ACTING AS CAPT ON FLT FROM NAS TO BWI. ACFT TYPE WAS AN MLG TCASII EQUIPPED. BTWN 15-20 MI FROM BWI WHILE BEING VECTORED BY APCH CTL FOR LNDG AT BWI WE WERE GIVEN A DSCNT TO 4000 FT. PASSING 5000 FT DSNDING WE RECEIVED A TCASII ADVISORY OF 'MONITOR RATE OF DSCNT.' AN ACFT SYMBOL WAS DISPLAYED IN OUR 10 O'CLOCK POS AT 3500 FT; 3 MI AND LEVEL. WHILE LOOKING FOR THE TFC VISUALLY; I HAD MY FO ASK APCH CTL ABOUT THE TFC. PRIOR TO THEIR RESPONSE; WE RECEIVED A 'CLB; CLB NOW' COMMAND ON TCASII. WE REACTED AS COMMANDED VIA TCASII TO CLB AT 1500 FPM. SINCE WE COULD NOT ATTAIN THE INTRUDER VISUALLY WE BEGAN A CLB AS PER FAA REG. WE NOTICED AT THIS TIME OUR ALT WAS 4300 FT DSNDING AND THE INTRUDER HAD STARTED A CLB. WE ADVISED ATC THAT WE WERE RESPONDING TO THE TCASII COMMAND AND ABOUT THE SAME TIME RECEIVED A 'CLR OF CONFLICT.' HIGHEST ALT OBTAINED WAS 4600 FT. WE NEVER DID SEE THE INTRUDER. ATC ADVISED THE VFR ACFT WAS NOT UNDER THEIR CTL. ATC ALSO ADVISED US THEY WOULD FILE AN INCIDENT RPT. 'PLT RESPONDING TO TCASII COMMAND.' WE THEN DSNDED TO AT CLRED ALT AND REMAINDER OF APCH WAS UNEVENTFUL.