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.
この事件では、主犯格が最も重い刑を受けるだろう。
In this case, the key culprit will likely receive the heaviest sentence.
主犯 is a more common term for the principal offender; 主犯格 adds a nuance of rank or status among offenders.
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.