B727 FLC INBOUND TO DCA RECEIVED A TCASII RA REQUIRING IMMEDIATE EVASIVE ACTION. ATC ADMITTED A B767 HAD BEEN GIVEN A DSCNT CLRNC THROUGH THE B727'S ALT.

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WE WERE ACFT X; LGA-DCA. INBOUND TO DCA VOR; 115 DEG RADIAL; 22 DME; LEVEL AT 10000 FT MSL; 25O KTS; HDG 295 DEGS; DC APCH 124.2 MHZ. DURING THE APPROX 15 SECONDS THIS EVENT ENCOMPASSED; WE WERE IN VMC IN DAYLIGHT BTWN CLOUDS INCLUDING CUMULO NIMBUS. I FOUND OUT LATER THAT THE OTHER ACFT WAS ACFT Y INBOUND TO BWI WORKING BAL APCH. OUR INITIAL INDICATION OF THE OTHER ACFT WAS A TCASII TA FOLLOWED IMMEDIATELY BY AN RA TELLING US TO DSND. ON MY TCASII INDICATOR THE SUBJECT ACFT WAS DIRECTLY IN FRONT OF US; 700 FT ABOVE US DSNDING TOWARDS US; INSIDE THE 2.5 MI RING ON THE INDICATOR. THEN SEVERAL THINGS HAPPENED ALMOST AT ONCE. I STARTED A DSCNT; LOOKED UP AND SAW THE ACFT; LOOKED DOWN AT THE TCASII INDICATOR AND SAW HE WAS 300 FT ABOVE US STILL DSNDING; LESS THAN 1 MI IN FRONT OF US; STARTED A L TURN; LOOKED UP AND SAW HIM IN WHAT APPEARED TO BE A PULL-UP AND TURN TO HIS L AS HE PASSED TO OUR R SIDE. I WOULD ESTIMATE WHEN HE WENT BY US HE WAS NO MORE THAN 100 FT ABOVE US AND ABOUT 500-1000 FT HORIZ AWAY. WE RPTED THE NEAR MISS TO ATC AND PROCEEDED TO A NORMAL ARR. SHORTLY AFTER ARR; I TALKED TO A SUPVR AT THE DCA TRACON. HE SAID THEY WERE INVESTIGATING THIS EVENT AS A 'SYS ERROR.' HE VERY CANDIDLY ADMITTED TO ME THAT THE ACFT Y FLT HAD BEEN GIVEN A DSCNT CLRNC THROUGH OUR ALT DUE TO MISCOM BTWN THE DCA AND BAL TRACONS.