When you are talking about a crime or something wrong that has been done, you can refer to the person who did it as the culprit .
例句
All the men were being deported even though the real culprits in the fight have not been identified.尽管真凶尚未查出,所有参与斗殴的男子均被驱逐出境。
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引起问题(或不利情势)的事物
When you are talking about a problem or bad situation, you can refer to its cause as the culprit .
例句
About 10% of Japanese teenagers are overweight. Nutritionists say the main culprit is increasing reliance on Western fast food.大约10%的日本青少年超重。营养学家指出主要原因是对西式快餐越来越依赖。