undercover agents：覆面警察官

To go "undercover" is to avoid detection by the entity one is observing, and especially to disguise one's own identity or use an assumed identity for the purposes of gaining the trust of an individual or organization to learn or confirm confidential information or to gain the trust of targeted individuals in order to gather information or evidence. Traditionally, it is a technique employed by law enforcement agencies or private investigators, and a person who works in such a role is commonly referred to as an undercover agent.(From Wikipedia, the free encyclopedia)

River and air patrols, bomb-sniffing dogs, and undercover agents are all on guard, looking for anything resembling a threat.：脅威となりそうなものを探しながら、川や上空のパトロール隊、爆弾探知犬、そして覆面警察官がみな警戒に当たっています。（by CBS News）