The man was said to be part of a rogue militant group.
I looked up “berandal” in several dictionaries and corpora such as KBBI, IndoDic, SEAlang, and Leipzig Corpora Collection. But none of them uses “berandal” as an adjective. I am wondering whether inserting “yang” between “militan” and “berandal” is grammatically correct or not.
“Berandal” is often used as in a combined noun such as “anak berandal” (young troublemaker), “kelompok berandal” or “para berandal” (gangsters), and “anggota berandal motor” (a member of bike gang). I guess it sounds more natural to remove “yang” from this sentence, and to rephrase it as “sebuah kelompok berandal militan” because “militan” is an adjective.