SIOUX FALLS, S.D. (AP) – The U.S. Department of the Interior on Tuesday announced that it has sent nearly 16,000 new offers to owners of parcels on the Pine Ridge Reservation in South Dakota as it tries to buy back land that was given to individual Oglala Sioux Tribe members more than a century ago and return it to the tribe as a whole.

The buyback program stems from the settlement of a nearly 17-year lawsuit over more than a century’s worth of mismanaged trust royalties. The 1887 Dawes Act split tribal lands into individual allotments – 80- to 160-acre parcels, in most cases – that have been passed down to multiple heirs. The Interior Department said the Pine Ridge Indian Reservation is among the most fractionated in the U.S., with land interests owned by various individuals, including members of other tribes.

The Interior Department is now trying to buy back those lands, consolidate them and hold them in trust for the tribe. The offers announced Tuesday are the second round the department has sent to the reservation and total more than $100 million. The buyback program is voluntary, meaning owners do not have to sell their so-called fractionated interests.

“Consolidating and returning these lands to tribes in trust will have enormous potential to unlock tribal community resources,” the department’s assistant secretary for Indian Affairs Kevin Washburn said in a statement. “While we know that it will be a challenge to reach all landowners, we are committed to exhausting all efforts to make sure that individuals are aware of this historic opportunity to strengthen tribal sovereignty by supporting the consolidation of tribal lands.”

According to Interior Department estimates released last year, there are more than 92,000 land tracts on tribal lands with 2.9 million fractional interests. Of that number, more than 21,200 land tracts have 100 or more owners and many parcels have thousands of owners.

Before the buyback program began in December, officials had estimated that Pine Ridge held more than 6,000 tracts with nearly 200,000 purchasable fractional interests, which had made it increasingly difficult to manage the land for economic development and other uses.

The cultural liaison for the land buyback program in the reservation, Maxine Broken Nose, on Tuesday said the first wave of lands that were sold between late December and mid-January gave a boost to the reservation’s economy. Payments to sellers during the first round of buy backs exceeded $10 million. The department said some individuals received more than $100,000.

Broken Nose said some individuals who sold their fractionated interest spent some of their money in the community and one even built a general store on the reservation.

The 10-year buyback program is the largest part of the $3.4 billion settlement of a class-action lawsuit filed by Elouise Cobell of Browning, Mont., in 1996 and finalized in 2012.