Felony crimes such as rape, serious assaults, fraud, theft, etc which occur within the boundaries of a federally recognized tribe are investigated by the FBI and if available, the BIA. In some cases the tribe has a Memorandum of Understanding, approved by the US Government to have the State Police conduct these investigations, i.e. tribes in Alaska.
These crimes, involving tribal members either as suspects or victims ARE NOT ignored. While the quality of the investigation varies by the quality of the Agent or the U.S. Attorney's Office, investigations/prosecutions are monitored by the FBI's Indian Country Unit and the DOJ.
Minor crimes are handled by the tribal police who will enact the Mutual Aid agreement mandated by law, to obtain outside police agencies assistance particularly regarding a non Indian suspect or victim.