Decided, Settled
noun, の adjective
It was previously decided, which means it's been decided and settled.
きけつ
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
あなたのケースでは、既決囚救済制度が適用される可能性もなきにしもあらずだと思います。
I think that applying for post-conviction relief may be a possibility in your case.