in an attack，明白了么？你要想确定一个介词，你要找它对应的宾语。
如果还不明白，你试着把句子换成... in an attack that was believed to be...，这样句子就很清楚了。what引导了一个名词性从句作介词的宾语，本质上就是attack。
你对从句没有理解透.
in which常在定语从句中出现.
如: The house in which we had lived for ages was destroyed
in the flood.
此句中的in which 就是定从. 修饰 the house. in which=where
in what 一般会在名词性从句中出现.
如: I am quite interested in what you want for your birthday.
what you want for your birthday为in的宾语.
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