Narrative:

Aircraft X checking in descending to 24000 feet. I continued to descend him to 14000 feet. The high altitude sector previously had MOA active 260b280 feet. [A military flight of 2] exited the airspace. However the controllers did not realize that [the military flight] was actually a flight of 2 and only aircraft Y departed. Aircraft Z remained in the MOA with their transponder off. The high sector declared MOA inactive and had no clue aircraft Z was still there. When the high sector controller realized there was still an aircraft in the MOA; they thought when they spoke to aircraft Z he was at 25000 feet. So they immediately told me to stop aircraft X at 26000 feet who was already descending out of 28800 feet. Unfortunately aircraft Z was actually at 27000 feet when aircraft Z actually began to squawk his beacon code. I called traffic immediately to aircraft X.he told me he got a TCAS alert but no RA. Prior to this incident while we thought the MOA was inactive; another aircraft departed going southeast bound and via coordination with the high altitude sector they allowed me to climb him to 27000 feet through the what we thought was inactive MOA. We don't think the aircraft ever got close but we are very uncertain of the situation.controllers to verify number of aircraft in a flight when entering a MOA. Controllers should update flight plan to flight of 2 if they knew he was a formation flight and they decided to drop the second track!

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Original NASA ASRS Text

Title: Center controllers report of a civilian aircraft receiving a TCAS alert from military aircraft operating within a mistakenly deactivated MOA. Confusion resulted from the misunderstanding that the military aircraft was not a single aircraft but rather a flight of two that had ceased joint operations and were subsequently operating independently.

Narrative: Aircraft X checking in descending to 24000 feet. I continued to descend him to 14000 feet. The high altitude sector previously had MOA active 260B280 feet. [A military flight of 2] exited the airspace. However the controllers did not realize that [the military flight] was actually a flight of 2 and ONLY Aircraft Y departed. Aircraft Z remained in the MOA with their transponder off. The high sector declared MOA INACTIVE and had no clue Aircraft Z was still there. When the high sector controller realized there was still an aircraft in the MOA; they thought when they spoke to Aircraft Z he was at 25000 feet. So they immediately told me to stop Aircraft X at 26000 feet who was already descending out of 28800 feet. Unfortunately Aircraft Z was actually at 27000 feet when Aircraft Z actually began to squawk his beacon code. I called traffic immediately to Aircraft X.He told me he got a TCAS alert but no RA. Prior to this incident while we thought the MOA was inactive; another aircraft departed going southeast bound and via coordination with the High altitude sector they allowed me to climb him to 27000 feet through the what we thought was inactive MOA. We don't think the aircraft ever got close but we are very uncertain of the situation.Controllers to verify number of aircraft in a flight when entering a MOA. Controllers should update Flight Plan to flight of 2 if they knew he was a formation flight and they decided to drop the second track!

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.