If this is an emergency, call 911. To report criminal activity, contact your local police or sheriff's department first as they can best determine if a report of criminal activity needs attention. The USAGov website offers additional tips and resources for reporting local or state law violations as well as violations of Federal law.
You may also report suspected violations of Federal law to the Federal Bureau of Investigation. Submit a tip at https://www.fbi.gov/tips or call 1-800-CALL-FBI (1-800-225-5324).