BILLINGS, Mont. — Federal and state regulators underestimated the potential for a toxic blowout from a Colorado mine, despite warnings more than a year earlier that a large-volume spill of wastewater was possible, an internal government investigation released Wednesday found.

The regulators wrongly concluded there was little or no pressure from the millions of gallons of water trapped inside the inactive Gold King mine, the federal Environmental Protection Agency concluded in its probe.

The massive spill occurred Aug. 5 when a government cleanup crew doing excavation work triggered the release of about 3 million gallons of sludge that fouled hundreds of miles of rivers in Colorado, New Mexico and Utah.

The torrent of toxic water released from the mine also shut down some public drinking water and irrigation systems.

“There was in fact high enough water pressure to cause the blowout,” EPA Deputy Administrator Stan Meiburg said after the release of documents. He said the error was likely the most significant factor behind the spill.

The EPA previously offered only partial information on events leading up to the spill.