Government investigators say the U.S. Environmental Protection Agency had no rules for working around old mines when the agency inadvertently triggered a massive spill from a Colorado mine that polluted rivers in three states.

The EPA's Office of Inspector General said Monday the agency started work on safety standards after the spill and expects to release them this week.

An EPA-led crew was excavating at the Gold King Mine in southwestern Colorado in 2015 when a debris pile collapsed, releasing 3 million gallons of wastewater tainted with heavy metals into rivers in Colorado, New Mexico and Utah. Native American tribes in those states were also effected.

State, tribal and federal officials have criticized the EPA for not taking more precautions, such as drilling into the mine to determine how much water was pent up inside.