The last uranium mine on Navajo land was shut down in 1986.

The U.S. government will put $13.2 million into an environmental trust to pay for evaluations of 16 abandoned uranium mines on land belonging to the Navajo Nation in Utah, Arizona and New Mexico, the Justice Department said on Friday.

The Justice Department said the agreement was part of its increased focus on environmental and health concerns in Indian country, “as well as the commitment of the Obama Administration to fairly resolve the historic grievances of American Indian tribes and build a healthier future for their people.”

The investigation of the sites is a necessary step before final cleanup decisions can be made, it said in a statement, adding the work would be subject to the approval of both the Navajo Nation and the Environmental Protection Agency.

“The site evaluations focus on the mines that pose the most significant hazards and will form a foundation for their final cleanup,” Assistant Attorney General John Cruden of the Justice Department’s Environment and Natural Resources Division said in the statement.

The Navajo Nation encompasses more than 27,000 square miles (70,000 square km) within Utah, New Mexico and Arizona.

The region’s unique geology makes it rich in uranium, a radioactive ore which has been in high demand since the development of atomic power and weapons at the close of World War Two.

Some four million tons of uranium ore were extracted during mining operations within the Navajo Nation from 1944 to 1986, the department of justice said.

The last uranium mine on the Navajo Nation was shut down in 1986.

Navajo Nation President Ben Shelly said in a statement he welcomed the agreement. “We have always said the U.S. is responsible for the cleanup of uranium legacy sites,” he said.