American Indians make up 9 percent of North Dakota’s population, but Indian children constitute nearly 30 percent of the state’s child abuse victims, a 2009 study by the Department of Health and Human Services found.

While statistics related to abuse at Spirit Lake are not public information, federal officials believe that the reservation has an even more significant child abuse problem than others in Indian Country. As one indication, the reservation is home to 38 registered sex offenders out of a population of 4,500, according to Justice Department figures — a far higher proportion than in most cities and towns in the United States.

The recent e-mails from two highly respected government officials to the health department’s Washington headquarters present a picture of a reservation hierarchy more intent on masking rampant child abuse than on ensuring the safety of children.

“The leadership of the Spirit Lake Nation as well as those who are responsible for delivering services on that reservation, by their actions as well as their inactions, have failed in their most basic responsibility to protect children,” Mr. Sullivan wrote. “They have hung signs at the borders of the Spirit Lake Nation, ‘Pedophiles Welcome.’ They have made these signs operational by firing professionally qualified staff, directing their replacements to ignore reports of abuse and neglect, refusing to prosecute the most egregious cases of abuse.”

While the tribe’s leaders have not actually placed such signs on Spirit Lake, a clearly exasperated Mr. Sullivan also called for Roger Yankton Sr., the tribe’s president, to be charged with a federal crime, saying Mr. Yankton has allowed children to continue to be harmed without addressing the problem.

The second e-mail, written in April by Michael R. Tilus, the director of behavioral health at the federally financed Spirit Lake Health Center, came below a subject line that read “Letter of Grave Concern.” Dr. Tilus said that child abuse on the reservation was “epidemic” and that he had “no confidence” in tribal leadership “to provide safe, responsible, legal, ethical and moral services to the abused and neglected children of the Spirit Lake Tribe.”

Tribal officials did not return phone calls seeking comment.

Mark Weber, a Health and Human Services Department spokesman, said the agency was working with the tribe and other agencies “to address concerns regarding the Spirit Lake Tribe’s Social Service Department.”