I am tempted to parse
全て手配中のもしくは刑務所に留置されていた犯罪者たちです
as
全て((手配中で)もしくは(刑務所に留置されていた))犯罪者たちです
meaning
All are criminals who (being searched for) or (have been detailed in prison).
But I replaced の with で (て-form of だ) because it doesn't make much sense to me otherwise.
Why is の being used here? Regardless, is there a copula being colloquially dropped after 手配中 (which I assume means "in-the-middle-of-being-searched-for")?