Southern Arizona Congressmen Raúl Grijalva and Tom O'Halleran have asked for a more formal commitment to sustaining a ban on uranium mining near the Grand Canyon, in a letter to U.S. Secretary of the Interior Ryan Zinke.
The letter said the Interior Department has made such a promise in tweets, but that the members of Congress want something more formal.
A 2012 moratorium put a 20-year ban on mining claims on land surrounding the park.
There are hundreds of abandoned mines on Navajo Reservation, near the Grand Canyon. The representatives said the clean-up of those mines has already cost more than $1 billion.
Representatives Ruben Gallego and Kyrsten Sinema also signed the letter.
UPDATE: This story was updated Aug. 2, 2018 to clarify the existence of a ban on uranium mining in the area that started in 2012.