NMAC OCCURS AFTER HDG TRACK DEV. SEE AND AVOID CONCEPT NOT EXERCISED.

Narrative

MY ACFT WAS CLRED FOR A VISUAL APCH TO RWY 21R AT DTW. THIS INVOLVED A R TURN IN FROM THE N TO THE RWY. WE WERE ISSUED TFC INFO ON 2 ACFT. ONE; THE TFC WE WERE TO FOLLOW TO 21R AND TWO; AN MLG 11 O'CLOCK 5 MI ON A L BASE TO RWY 21C. INITIALLY; I SAW THE TFC I WAS TO FOLLOW. I THEN LOOKED TO FIND THE SECOND ACFT. I SAW AN ACFT THAT I BELIEVED WAS THE 11 O'CLOCK TFC AND WHEN I LOOKED BACK I WAS UNABLE TO SEE THE ACFT I WAS TO FOLLOW. MY ATTN WAS DIVERTED WHILE LOOKING FOR THE ISSUED TFC AND I OVERSHOT THE EXTENDED CTRLINE FOR 21R. BEFORE I COULD REACT TO THE OVERSHOOT; AN ACR MLG Y APPEARED TO MY R AND 300 FT OR SO BELOW OUR ACFT. IT WAS OBVIOUS THAT HE HAD JUST PASSED UNDERNEATH US ON WHAT WOULD HAVE BEEN A COLLISION COURSE IF WE HAD BEEN AT THE SAME ALT. I WAS IN A SLIGHT R TURN AND THE MLG APPEARED TO BE WINGS LEVEL OR NEARLY SO. HOW CLOSE WE ACTUALLY CAME WHEN HE PASSED BENEATH ME I DO NOT KNOW. I DO NOT KNOW IF THE MLG WAS IN FACT THE ISSUED TFC I SAW AND ASSUMED TO BE THE ISSUED COULD NOT HAVE BEEN THE ONE WITH WHICH WE HAD JUST HAD OUR 'CLOSE ENCOUNTER.' AGAIN; HIM TO LAND ON 21C AND ME TO LAND ON 21R.