How to write a mystery when the detective knows who did it from the start?

Hello!

So I'm currently working on a mystery novel for the first time, and I'm a bit stuck lol. So it follows a young detective with a secret past. When he was a child he was taken by a serial killer but survived. He was the only victim that did. The killer was never arrested and went underground. Years go by, when suddenly the MC's niece goes missing, and all clues point to the one who took her being the same killer that tried to kill the MC when he was her age. The MC must now revisit and work through his old trauma and personal experiences with the killer if there's any hope of finding her.

So essentially the audience (plus the characters) already know who the culprit is. The main plot is finding out where he is, rather than finding out who he is. Thing is I've never written a mystery before, so I'm not sure how to piece together all of the clues, red herrings, and what not. Especially since the story isn't the usual whodunnit. Everywhere I look, all of the advice is centered around finding out who did it, but that's not very helpful for my story lol.

Any advice on piecing together a mystery like this (or just a mystery in general) would be greatly appreciated!