You use who after certain words, especially verbs and adjectives, to introduce a clause where you talk about the identity of a person or a group of people.
双语例句
例： Police have not been able to find out who was responsible for the forgeries.
警察还未能发现谁是那些伪造案的元凶。
例： I went over to start up a conversation, asking her who she knew at the party.
我走过去和她攀谈，问她在聚会上都认识谁。