The U.S. Code of Federal Regulations defines terrorism as "the unlawful use of force and violence against persons or property to intimidate or coerce a government, the civilian population, or any segment thereof, in furtherance of political or social objectives" (28 C.F.R. Section 0.85).
From police murders of unarmed to civilians to violent suppression of political dissent to civil asset forfeiture, the actions of police across the United States clearly meet the criteria for classification as a terrorist organization.