NMAC BTWN AN ACR AND AN UNKNOWN ACFT.

Narrative

DURING COM WITH BWI APCH; TFC RPTED AT 1 O'CLOCK; UNKNOWN ALT. TCASII ALERT ISSUED TO DSND. ACFT DSNDED TO 11500 FT. AT THAT ALT; TFC WAS SEEN DIRECTLY OVERHEAD. APCH CTL WAS NOTIFIED AND ACTED AS IF UNAWARE OF TFC. WITHOUT COMMENT; HE HANDED US OFF TO NEXT CTLR. BETTER HANDLING AND AWARENESS BY ATC MAY HAVE AVERTED WHAT COULD HAVE BEEN A DISASTER. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR FLIES THE MD80. THERE WAS GOOD VISIBILITY AT THE TIME OF THIS INCIDENT. THE RPTR BELIEVES THAT HIS TCASII WAS OPERATING PROPERLY; THAT THIS WAS NO FALSE ALARM. 'THIS WOULD HAVE BEEN REALLY CLOSE IF NOT A HIT' EXCEPT FOR THE TCASII WARNING. HE IS UPSET WITH THE RATHER CAVALIER ATTITUDE TAKEN BY THE ARTCC CTLR WHO OFFERED NO EXPLANATION OR APOLOGY; JUST A FAST HDOF TO THE NEXT SECTOR. THE RPTR DOES NOT REMEMBER WHAT TYPE ACFT WAS OVERHEAD; BUT IT WAS NOT AN ACR. THE 500 FT ALT DIFFERENTIAL WAS AN ESTIMATE. INITIALLY THE TCASII SHOWED THE INTRUDER TO BE AT THE SAME LEVEL AND THE RPTR DSNDED 500 FT. HENCE THE ESTIMATE. THE RPTR BELIEVES THAT THE INTRUDER DID NOT HAVE ALT RPTING CAPABILITY.