The Environmental Protection Agency, which accidentally spilled three million gallons of toxic wastewater from an abandoned mine in Colorado, lacks the technical skills to handle such tricky projects, a government assessment reported on Thursday. The big accident could have been averted if the agency had had greater expertise and acted more prudently, the report said.

The E.P.A. had intended to gradually drain and capture contaminated water that was seeping out of the Gold King Mine and poisoning local streams in the area, which is high in the Rocky Mountains in the southwestern part of the state. Instead, on Aug. 5, workers caused a blowout, releasing a yellow-red torrent into Cement Creek and then into the Animas River. The agency has conceded that its people underestimated the volume of water collected in the mine and how much pressure it was under. It requested a separate inquiry by the Interior Department, which released its 132-page report by a team of engineers on Thursday.

At the E.P.A., the report said, “abandoned mine guidelines and manuals provide detailed guidance on environmental sampling, waste characterization and water treatment, with little appreciation for the engineering complexity of some abandoned mine projects that often require, but do not receive, a significant level of expertise.”

In particular, it said, the E.P.A. does not adequately “analyze the geologic and hydrologic conditions of the general area” or comprehend how, in a region honeycombed with tunnels, changing conditions in one affect the others.