The U.S. Supreme Court on Monday declined to hear arguments in a case between New Mexico and Colorado stemming from the devastating 2015 Gold King Mine spill.

The New Mexico Attorney General’s Office and the state Environment Department announced last year that it filed a complaint against Colorado with the U.S. Supreme Court. It sought damages and demands that Colorado address problems at draining mines in southwest Colorado.

Former New Mexico Environment Department Secretary Ryan Flynn alleged that his water quality researchers rejected assertions from Colorado Gov. John Hickenlooper and Colorado environment officials that the Animas River quickly returned to safe pre-event conditions after the August 2015 spill of toxic heavy metals.

Flynn and attorneys for his department at the time suggested that Colorado is liable for the incident, which spilled 3 million gallons of sludge into the Animas in Durango, turning it a mustard yellow color. The spill fouled rivers in three Western states with arsenic, lead and other heavy metals.

The EPA acknowledged fault in the spill, in which sludge flowed into creeks and rivers during restoration work at Gold King. The flow headed into the San Juan River in New Mexico and Utah.

The EPA employed contractors whose work trigged the incident. The agency acknowledged fault, which it said was the result of insufficient planning during excavation work at the entrance to the mine near Silverton. Debris gave way after the EPA team failed to properly assess pressure inside the mine.

“Because it was the EPA and not Colorado that caused the Gold King Mine disaster, I have said from the beginning that New Mexico should not have sued Colorado in the Supreme Court,” Republican Colorado Attorney General Cynthia Coffman said in a statement following the Supreme Court’s announcement. “Now that my office has won the Supreme Court case, I hope the conversation can focus on the EPA and its promise to take full responsibility for its actions.”

Colorado officials with the Department of Natural Resources have maintained that its Division of Reclamation, Mining and Safety was never on board with the EPA’s restoration plan. But an internal investigation by the EPA determined that the Colorado Division of Reclamation, Mining and Safety agreed to put drainage piping through the entrance of the mine, contributing to the spill.

During the spill, water utilities shut down intake valves and farmers stopped drawing from the rivers as the plume moved downstream.

The Supreme Court was an appropriate venue for the case against Colorado, as it involved two states suing each other. But the high court declined to hear arguments in the case, though it did not issue an opinion explaining the decision. Justices Clarence Thomas and Samuel Alito said they would let the lawsuit move forward.