B727 FLC EXECUTES EVASIVE CLB FROM FL310 DUE TO TCASII RA FROM AN ACFT CLBING BELOW THEM. THIS PUTS THE B727 IN CONFLICT WITH A DC8 AT FL330. ATC ADVISES THEY ARE NOT CTLING ANY TFC BENEATH THE B727.

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DEPARTED MSP ARPT AND CLBED TO FL310. GIVEN A VECTOR DIRECT ONL FROM ZMP. AT APPROX XA54Z; A TCASII TA WAS RECEIVED WITH ACFT Y AT OUR 3 O'CLOCK POS; HALF OFF THE SCREEN. ABOUT 3-8 SECONDS LATER; A TCASII RA WAS GIVEN WITH THE COMMANDS TO 'CLB; CLB NOW.' THE FO BEGAN A CLB WHILE THE CAPT PUSHED THE PWR LEVERS FORWARD. AS WE WERE IN A 1500-2000 FPM CLB; I SAW ON THE TCASII SCREEN ACFT Y BELOW US AND ALSO CLBING. 500 FT BELOW US; THEN 400 FT BELOW AS IT WENT UNDER OUR ACFT. THIS WAS CONFIRMED BY AN OBSERVER WHO SAW ACFT Y PASS UNDER OUR ACFT OR JUST SLIGHTLY DOWN THE R SIDE. THE FO RPTED HE SAW 2 ACFT ON THE TCASII SCREEN; 1 BELOW AND 1 ABOVE. WE CONTACTED ATC AND RPTED A CLB; THEN LEVELED OFF AT FL317. ATC SAID THEY ONLY HAD 1 ACFT ON THEIR EQUIP AND IT WAS AT FL330 AT OUR 3 O'CLOCK POS (ACFT Z). A 'CLR OF CONFLICT' AURAL MESSAGE WAS GIVEN ABOUT 3 SECONDS AFTER ACFT Y WENT UNDER US. WHAT I BELIEVE HAPPENED IS THAT ACFT Z AT FL330 WAS HDG OUTBOUND ON THE FSD 124 DEG RADIAL; AND AS WE CROSSED THE FSD 124 DEG RADIAL HDG DIRECT TO ONL; ACFT Z AT FL330 WAS AT OUR 3 O'CLOCK POS. A SHORT TIME AFTER IS WHEN WE RECEIVED OUR TCASII TA AND RA. I THINK ACFT Y; BELOW US; WAS TRYING TO INTERCEPT ACFT Z AT FL330 AND DID NOT SEE OUR ACFT (X). THE ONLY THING THAT DOES NOT MAKE SENSE IS THAT ATC SAID THEY DID NOT HAVE THE ACFT BELOW US ON HIS SCREEN. WITHOUT ALL THE FACTS; IT ONLY SEEMS THAT POSSIBLY A MIL ACFT COULD HAVE BEEN ON MANEUVERS TO INTERCEPT. SUPPLEMENTAL INFO FROM ACN 397300: ATC WAS ADVISED OF OUR MANEUVER AND SUBSEQUENTLY RELAYED THAT 'THE ONLY TFC WE KNOW OF' IS A FREIGHTER AT FL330. WE RETURNED TO FL310 AND RESUMED NORMAL FLT.