The San Juan River flows through the Navajo Reservation, Wednesday, Aug. 12, 2015, in Shiprock, NM. Toxic wastewater from the Gold King Mine in Silverton, Colo., has contaminated the San Juan River in northern New Mexico.

(AP Photo/Matt York)

The Navajo Nation has formally endorsed a Superfund cleanup of southwestern Colorado mines, including one that released millions of gallons of wastewater into a river on Navajo land.

The U.S. Environmental Protection Agency, which is considering a Superfund designation for the Gold King Mine and other sites, released the letter Monday.

The letter was dated June 3. The EPA didn't immediately respond to a request for an explanation for the delay.

An EPA-led crew accidentally caused a 3-million-gallon spill at the Gold King last August while doing preliminary cleanup work. The release polluted rivers in Colorado, New Mexico and Utah, including the San Juan River on Navajo land.

A Superfund designation could release millions of dollars for a cleanup. The EPA says a decision could come as early as this fall.