The higher up the governmental chain, the more sophisticated the level of technology and the greater the resources brought to bear in the surveillance of potential criminals and terrorists. In fact, with regard to the latter category, the federal government is always involved, as laws intended to combat terrorism exist primarily at the federal level, and the federal government becomes actively involved in investigations of potential terrorist activity through local FBI field offices. After the terrorist attacks of September 11, 2001, the government established a network of Joint Terrorism Task Forces in which the FBI works closely with local law enforcement to investigate possible terrorist-related activities. These investigations involve not only the questioning of suspects and those around whom the suspects travel and meet, but also extensive surveillance.
At the local level, law enforcement agencies lack the financial resources needed for the kind of surveillance operations associated with criminal activity and terrorism. City police departments and county sheriff's departments, depending upon their size (the New York City Police Department and its Los Angeles counterparts enjoy considerably greater resources than most departments) may be able to afford, in terms of manpower and money, to maintain continuous surveillance on suspects, but it is difficult and rare.
State police departments are often better equipped to conduct long-term surveillance of suspects, and are often called in to assist local agencies. At the top of the resource pyramid is the FBI, which has developed tactics and technologies for surveillance purposes that are second to none.
The first, and best means of attaining intelligence on suspects is through the infiltration of an undercover officer into the criminal or terrorist group. Some of the best intelligence comes from undercover police officers who witness criminal conduct or the planning of attacks by terrorists, and can also later testify on their observations during a criminal trial. Next best is the use of informants, usually criminals or potential terrorists who keep law enforcement informed on the activities of their colleagues or friends in exchange for leniency in their own criminal cases.
The FBI has gotten very good at planting listening devices in the homes of criminal suspects and potential terrorists, as well as at the bars where they hang-out, restaurants they frequent, and so on. This type of surveillance, which requires a warrant issued by a judge, has been extremely effective at recording criminal conspiracies for use in prosecutions. Many organized crime figures have been successfully prosecuted due to a combination of recorded conversations and testimony by informants.
The government is relying on U.S. intelligence agencies to assist in the conduct of surveillance on possible terrorists. Recent revelations about the National Security Agency's role in tracking suspects through massive data collection operations have worried many Americans about possible illegal intrusions into their privacy as the government casts ever-wider surveillance nets seeking out terrorists.
Instances of illegal wiretapping by city police, especially in New York during the 1970s, have occurred, but they are rare. Law enforcement intrusions into privacy mainly occur at the federal level, and are being monitored by civil libertarians and nongovernmental legal organizations.