How about the start with the Federal agencies that have their own SWAT or law enforcement divisions.
Do the Department of Agriculture, the Railroad Retirement Board, the Tennessee Valley Authority, the Office of Personnel Management, the Consumer Product Safety Commission, and the U.S. Fish and Wildlife Service all need a SWAT?
Does the IRS, EPA, National Park Service, Department of Education, and Food and Drug Administration really need a law enforcement division?
Start there and then we can talk about state and local police.