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.