In investigating criminal activity, as well as in gathering intelligence on suspected criminal or terrorist organizations, similar types of surveillance techniques are used by law enforcement agencies at all levels of government.
The most important type of surveillance is also the most potentially controversial: wiretapping and eavesdropping on conversations. Short of DNA linking a suspect to a crime, the most important component of an investigation or intelligence-gathering operation involves electronic surveillance, especially of voice communications. When investigating a criminal organization and preparing a prosecution, recorded conversations between suspects are invaluable. It is extremely difficult for defense attorneys to refute legally-attained recordings of conversations between suspected members of organized crime, including ethnic-based groups like La Cosa Nostra and the Russian Mafia, involving discussions of criminal activities like drug trafficking, extortion, and corruption of public officials.
Another important type of surveillance used in both criminal investigations and in gathering intelligence on potential suspects involves the use of informants. Well-placed informants who can accurately relate the substance of conversations between suspects or, even better, surreptiously record those conversations, are extremely valuable to prosecutors. Informants are not, however, as reliable as conversations recorded directly by law enforcement agencies through wiretaps or electronic eavesdropping. Often, informants are themselves criminals or associates of criminals who have made agreements with law enforcement agencies to help gather incriminating evidence on their associates in exchange for leniency in their own criminal trials. Such arrangements allow defense attorneys to try to undermine the informant's credibility during the trial.
Better than an informant in both helping build a criminal case and gathering intelligence on a potential criminal conspiracy or terrorist plan is the successful infiltration into the criminal or terrorist organization of an undercover law enforcement officer. While extremely dangerous, the information gathered by such individuals is important for preventing terrorist attacks (assuming the agent's information reaches the necessary people and is acted upon) and for gathering evidence of a crime or conspiracy to commit a crime.
Photographic surveillance is another component used in both criminal investigations and in intelligence gathering. The Federal Bureau of Investigation routinely, and successfully, uses photographic surveillance to gather intelligence on organized crime members. Such photographs help to document the relationships between individuals, key to a racketeering prosecution directed against an organization in addition to individuals.