noun
key culprit; principal offender
Legal term for the main perpetrator in a crime, as opposed to accomplices or accessories.
警察は主犯格の男を逮捕した。
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
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.