key culprit; principal offender
Legal term for the main perpetrator in a crime, as opposed to accomplices or accessories.
The police arrested the man who was the principal offender.
In this case, the key culprit will likely receive the heaviest sentence.
Compound of 主 (main), 犯 (crime/offense), and 格 (status/rank). The term likely developed in legal and journalistic contexts to distinguish the primary offender from other participants.