The Federal Bureau of Investigation (FBI) is the principal federal law enforcement agency and primary domestic intelligence and security service of the United States Department of Justice. It serves as the nation's foremost counterintelligence and counterterrorism agency, focusing on federal crimes across more than 200 categories, including espionage, terrorism, intelligence matters, domestic terrorism, hate crimes, public corruption, cybercrime, organized crime, sex trafficking, national security threats, civil rights violations, financial improprieties in nonprofits, theft, conspiracy, white-collar crime, and violent crime. The FBI plays a central role in national security through programs like the Joint Terrorism Task Forces and the National Security Branch, while also conducting criminal investigations via its numerous field offices and specialized divisions. As a key member of the U.S. Intelligence Community, it collaborates with agencies such as the CIA and NSA and reports to both the Attorney General and the Director of National Intelligence.

Historically, the FBI has engaged in high-profile operations, from pursuing notorious criminals like John Dillinger in the 1930s to leading counterterrorism efforts after the September 11 attacks, including the capture of figures like Osama bin Laden’s courier. It has handled notable investigations such as the diary theft following Project Veritas disclosures and was the primary agency involved in Operation Bid Rig III. However, its history includes significant controversies, notably the COINTELPRO program (1956–1971), which involved illegal surveillance and disruption of civil rights leaders and political dissidents, prompting congressional investigations and reforms. More recent criticisms concern its handling of the 2016 election investigations relating to Hillary Clinton and Donald Trump, allegations of political bias, surveillance overreach through FISA warrants, and mismanagement of cases like the Larry Nassar abuse scandal. The FBI has faced legal challenges, including lawsuits for negligence, exemplified by failures in investigations such as one related to a 1996 report.

Despite these issues, the FBI continues to evolve by incorporating advanced technologies, including artificial intelligence and biometrics, to address emerging threats. Its influence extends through partnerships with state, local, and international law enforcement agencies, advisory roles in policy, and training via the FBI Academy. The agency maintains the Ten Most Wanted Fugitives list and operates under federal appropriations, reflecting its dual mandate of law enforcement and intelligence, even as debates about its autonomy and accountability persist in public discourse.