The United States attorney for South Dakota said Tuesday that prosecutors would re-examine the circumstances surrounding dozens of deaths that occurred on or near the Pine Ridge Indian Reservation, many dating back to the 1970s when the reservation was embroiled in political violence.

The Oglala Sioux tribe has for years sought a Justice Department review of the deaths. In May, tribal leaders sent a letter to Brendan Johnson, the United States attorney for South Dakota, asking that he direct the Federal Bureau of Investigation and the Bureau of Indian Affairs to reopen investigations into 28 deaths — many of which the original investigators had determined were caused by accidents or suicides.

On Tuesday, Mr. Johnson said that a team of three assistant United States attorneys, including Mr. Johnson’s top deputy, would review a total of 50 Pine Ridge-related deaths that had occurred during the past 40 years. Once the panel writes a report for each case, Mr. Johnson said, he will decide whether there are enough unanswered questions to ask the F.B.I. or another investigative agency to assist.

Prosecutors said Tuesday that they had already begun assembling case files, death certificates and other documents, and that they planned to meet with family members as the work continued.