People kayak in the Animas River near Durango, Colo., on Aug. 6, 2015, in water colored from a mine waste spill. (Jerry McBride/The Durango Herald via AP)

U.S. officials responsible for the Aug. 5 spill of toxic mine waste in southwestern Colorado had no plan in place for dealing with a catastrophic breach of the kind that fouled a long stretch of the state’s Animas River, an internal inquiry has concluded.

No one, from the local contractor to federal overseers in Washington, saw warning signs of a dangerous build-up in water pressure inside the Gold King Mine, which discharged 3 million gallons of liquid waste when an earthen wall collapsed as cleanup work was underway, investigators said in the report released Wednesday.

The Environmental Protection Agency deployed experienced experts to the mine site and followed a plan that had been vetted by outside technicians, none of whom raised significant concerns, the EPA-appointed investigators said. Yet, the agency’s team lacked crucial information, including a reliable estimate of the volume of water inside the abandoned mine, the report said.

“Given the maps and information known about this mine, a worst-case scenario estimate could have been calculated and used for planning purposes,” the report stated. As it was, the EPA’s team was “lacking emergency protocols in the case of a significant flow or blow out,” the document said.

[Here’s what a 3-million-gallon mine waste spill looks like.]

The report is a distillation of the EPA’s initial, three-week inquiry into the spill, which turned the Animas River mustard-yellow for dozens of miles and caused a temporary spike in levels of arsenic and other contaminants. EPA officials accepted responsibility for the spill, which occurred as a contract crew was performing work at the century-old mine near Silverton, Colo. EPA and Colorado state environmental officials had been seeking stop a slow leak of toxic metals from the mine into local waterways when the incident occurred.

EPA officials accepted the report’s conclusions while noting that it is not clear whether the pressure build-up inside the mine could have been detected. Data collected in the weeks before the spill gave no hint of problems, and more expensive drilling tests to detect high water pressure were impractical because of the mine’s location in a remote area on steep and unstable slopes.

“Certain precautionary steps were taken based on the conditions and information they knew,” Mathy Stanislaus, the EPA’s assistant administrator for the Office of Solid Waste and Emergency Response, told reporters. “While additional steps could have been taken under some circumstances, it’s not sure that [those] could have identified the blow-out conditions.”

The EPA report recommended new guidelines to assess risk levels at the scores of other abandoned mine sites that could be subject to government clean-up in the future. It also called for more contingency planning, to include protocols for dealing with a blowout “at those mine sites where there is a potential for such an event to occur.”