infiltrator
noun [ C ] uk/ˈɪn.fɪl.treɪ.tər/ us/ˈɪn.fɪl.treɪ.t̬ɚ/
卧底人员
a person who secretly becomes part of a group in order to get information or to influence the way the group thinks or behaves
The infiltrator was identified and killed. 那个卧底身份暴露后被杀害了。