Indian reservations have for years grappled with chronic rates of crime higher than all but a handful of the most violent U.S. cities. But the Justice Department, which is responsible for prosecuting most violent crimes on reservations, files charges in only about half of Indian Country murder investigations and turns down nearly two-thirds of sexual assault cases, according to new federal data.

The country's 310 Indian reservations have violent crime rates that are more than 2 1/2 times higher than the national average, according to data compiled by the Justice Department. American Indian women are 10 times as likely to be murdered as other Americans. They are raped or sexually assaulted at a rate four times the national average, with more than 1 in 3 having either been raped or experienced an attempted rape.

The low rate of prosecutions for these crimes by U.S. attorneys, who along with FBI agents have jurisdiction on most reservations, has been a longstanding point of contention for tribes who say it amounts to a second-class system of justice that encourages lawbreaking. Prosecutors, however, say they turn down most reservation cases because of a lack of admissible evidence.

Brendan Johnson, the U.S. attorney for South Dakota, said the government in recent years has deployed extra prosecutors and FBI agents to tribal areas. And the Justice Department says it is seeking to make its decisions more transparent.

But tribes say they are rarely told why reservation cases are not pursued by the government.

Under federal law, tribal courts have the authority to prosecute tribal members for crimes committed on reservations but cannot sentence those convicted to more than three years in prison. As a result, tribes may seek federal prosecution.

Frustration has grown so acute that some tribal members have taken the unusual step of suing the government for declining prosecutions and for what they say is the related issue of sloppy police work.