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ringleader; main culprit
main cause; source
調査の結果、事故の元凶は部品の欠陥だと判明した。
The investigation revealed that the faulty part was the root cause of the accident.
刑事たちはついに連続強盗事件の元凶を特定した。
The detectives finally identified the ringleader of the serial robberies.
敗北の元凶は意思疎通の欠如だ。
The root cause of the defeat is a lack of communication.