Most of the spying has been done by the FBI but in recent years a whole alphabet soup of agencies (including the NSA, DIA, NRO, OICI, CIA) have been created and tasked with spying. Lots of that involved domestic spying for the simple reason it is easier and cheaper.
|Current and planned military drone bases inside the United States.|
The Pentagon has scores of domestic surveillance drone bases. The bases are officially for training but they can and do "accidentally" collect intelligence on US residents. Since the Pentagon routinely lies about its domestic drone activities it is likely commanders are not very resistant to the urge to watch over people they don't like.
Like the military, the CIA is forbidden by law to spy on American soil. Like the military, they ignore the law whenever they feel like it. The CIA has secretly tested drugs on Americans (Project MK-ULTRA). Born out the fear the Soviets were ahead of the US in brainwashing technology, the CIA began a decade long program of selecting unwitting people at random to test mind altering drugs on them. The only thing that came of it was to convert LSD from a minor therapeutic medicine to wide spread street drug.
Operation Chaos, whimsically named after the bad guys from the TV show Get Smart, ended up creating the men who committed Watergate. For a long time the CIA was jealous of the FBI's right to spy on Americans so they formed their own domestic program. During the Vietnam War, President Johnson tasked the CIA with spying on anti-war activists. They did everything but targeted assassinations although there are many people who believe it was the CIA domestic program that murdered President Kennedy.
The CIA is still at it. It was revealed in 2011 that the CIA recruited the New York Police Department to do a joint, and totally illegal, spying operation that probably has a cute code name that is still top secret.
The granddaddy of domestic spying agencies. The FBI has been spying on American citizens since the J. Edgar Hoover days. Hoover wiretapped every president from FDR to Richard Nixon. He began wiretapping the Supreme Court in the mid-1930's. The FBI spied on civil rights organizations, student and anti-war groups during the Vietnam War, and anyone ever vaguely suspected of being friendly to communism.
But the FBI didn't just spy. Their CONINTELPRO program went out of its way to defame people they didn't like. Actress Jean Seberg supported the civil rights movement. That led Hoover to conclude that she must be a "sex pervert." The FBI fabricated rumors about her and harassed her until she committed suicide. The program's criminal activity was revealed in 1971 and a decade later Ronald Reagan issued a executive order legalizing their crimes.
The programs, under other names, continue to this day.
people being led by FBI informants into committing crimes, sometimes doing all the work for the suspects. Other times the informants just make up shit to justify their pay.
The Occupy movement was replete with agent provocateurs sent in by the FBI with the specific goal of committing crimes to discredit the citizen protesters.