noun
principal offender; main culprit
Legal term for the primary perpetrator of a crime, as opposed to an accomplice or accessory.
警察は主犯者を逮捕した。
The police arrested the principal offender.
この事件の主犯者はまだ逃走中だ。
The main culprit in this case is still at large.
犯人 is a general term for a criminal or culprit, not specifying the degree of involvement.
Compound of 主 (shu, 'main') + 犯 (han, 'crime') + 者 (sha, 'person').