SPIRIT LAKE INDIAN RESERVATION, N.D. — The man who plays Santa Claus here is a registered child sex offender and a convicted rapist. One of the brothers of the tribal chairman raped a child, and a second brother sexually abused a 12-year-old girl. They are among a number of men convicted of sex crimes against children on this remote home of the Spirit Lake Sioux tribe, which has among the highest proportion of sex offenders in the country.

Federal officials are now moving to take over the tribe’s social service programs, according to members of the tribe, government officials and documents. The action comes after years of failure by government and tribal law enforcement officials to conduct proper investigations of dozens of cases of child sexual abuse, including rape.

While members of the tribe say that sexual violence against children on the reservation is common and barely concealed, the reasons for the abuse here are poorly understood, though poverty and alcohol are thought to be factors. The crimes are rarely prosecuted, few arrests are made, and people say that because of safety fears and law enforcement’s lack of interest, they no longer report even the most sadistic violence against children. In May 2011, a 9-year-old girl and her 6-year-old brother were killed on the reservation after being raped and sodomized.

“It bothers me that it is so accepted, that it is considered so normal. It’s lawless,” said Molly McDonald, who was a tribal judge until March, handling juvenile cases.