MLG CREW HAD TCASII RA IN IAD CLASS E.

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WHILE HOLDING AT CSN GOT A TCASII TA. NOTICED INTRUDER ACFT ON TCASII APPROX 500 FT BELOW; CLBING AND CLOSING AT 12 O'CLOCK POS. AS ACFT CLOSED AND CLBED; RECEIVED A TCASII 'CLB' RA. MY ALT WAS 4000 FT WITH ACFT HOLDING OVERHEAD AT 5000 FT AND 6000 FT. UNABLE TO NOTIFY APCH DUE TO FREQ CONGESTION AND I WAS NOT NOTIFIED OF INTRUDER ACFT. COPLT WITH MY CONCURRENCE INITIATED CLB IN RESPONSE TO TCASII INSTRUCTIONS. MY SCAN WAS PRIMARILY OUTSIDE LOOKING FOR INTRUDER ACFT AND TFC HOLDING ABOVE US. 'CLR OF CONFLICT' RECEIVED AT APPROX 5300 FT MSL. I DIRECTED COPLT TO IMMEDIATELY DSND TO CLRED ALT. FIRST OPPORTUNITY FOR CONTACT WITH ATC WAS DSNDING THROUGH 4500 FT MSL. CTLR INQUIRED AND I ADVISED HER WHY WE WERE FORCED TO LEAVE OUR ASSIGNED ALT. SHE ADVISED ME OF THE DANGER OF CLIMBING THROUGH OTHER HOLDING TRAFFIC. I ADVISED HER OF THE DANGER OF AN IMPENDING MIDAIR COLLISION. ACFT ABOVE US RPTED 5000 FT. THERE AFTER THAT INTRUDER ACFT HAD CAUSED A TCAS ALERT FROM SAME INTRUDER ACFT. CTLR STATED THAT SHE WAS ADVISING SUPERVISOR. REMAINDER OF FLT WAS UNEVENTFUL. I WOULD RECOMMEND THAT A PROC BE ESTABLISHED WHEREBY WHEN HOLDING BELOW 10000 FT AGL; 2000 FT SEPARATION BE CONSIDERED BETWEEN HOLDING ACFT VICE 1000 FT.