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")?
Nothing is omitted in your sentence. Your sentence is an example of right-node raising. The word 犯罪者 has been pulled out to the right.
So this の still modifies 犯罪者. Both mean the same, but the latter is more concise because you don't have to say 犯罪者 twice.
In case you still don't understand the former, remember that ～中 works as a no-adjective.
The sentence has a coordination of two parts in the middle (the parts in the two parentheses below). Both modify 犯罪者たち (as in 手配中の犯罪者たち and 刑務所に留置されていた犯罪者). This structure is possible because the forms Xの and Yていた are both modifiers of a noun.
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
It basically means that all of the (depicted?) suspects/criminals are wanted by police or previously jailed.
In speech you will usually have a pause before もしくは. You might add punctuation to make the structure clearer (but you might omit it for the lack of space).
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