But the fact is, while these tactics may never have been used in the investigation of a presidential candidate before, they are hardly unusual. Indeed, they are common procedures when undertaking covert public corruption investigations.

Undercover investigators usually take one of two possible forms — a confidential human source or an undercover agent. Confidential human sources are ordinary citizens who act as information gatherers for law enforcement organizations like the F.B.I. They have ordinary lives and live openly in the community, working on the side to assist law enforcement investigations. Their conduct is governed by extensive F.B.I. policies. Undercover agents are trained law enforcement personnel who adopt false identities to keep sensitive investigations secret. The use of undercover agents in any investigation is also subject to strict regulations and ones involving possible criminal conduct by domestic government officials or foreign governments must be approved by F.B.I. headquarters.

Both confidential human sources and undercover agents make contact with subjects of an investigation and elicit information without revealing their connection to law enforcement. They are key in counterintelligence investigations because they can help collect real-time information about the activity of foreign powers that individuals might be wary to share with investigators.