Dangerously high levels of water pressure behind the collapsed opening of the Gold King Mine were never checked by the Environmental Protection Agency, in part because of cost and time concerns.

The revelations came Wednesday as the EPA released an internal review of a massive Aug. 5 blowout at the mine above Silverton. The report called an underestimation of the pressure the most significant factor leading to the spill.

According to the report, had crews drilled into the mine’s collapsed opening, as they had done at a nearby site, they “may have been able to discover the pressurized conditions that turned out to cause the blowout.”

The EPA-triggered wastewater release sent yellow-orange sludge cascading through three states and the land of two American Indian tribes. The internal review for the first time reveals what the EPA believes went wrong at Gold King, which 14 months before the spill they knew was at risk for blowout.

“It is not evident that the potential volume of water stored within the (mine’s opening) had been estimated,” the review said. “Given the maps and information known about this mine, a worst-case scenario estimate could have been calculated and used for planning purposes.”

According to the review, drilling into the collapsed opening would have been “quite costly” and taken more time because of soil and rock conditions at the site.

The review says crews believed that because water was leaking from the Gold King and based on seep levels above its opening, a buildup of pressure was “less likely.” Because of those signs, officials say, drilling appeared to be unnecessary.

“The mine was draining,” Mathy Stanislaus, EPA assistant administrator in the office of solid waste and emergency response, said Wednesday during a media conference call.

EPA supervisor Hays Griswold, who was at the scene of the blowout Aug. 5, told The Denver Post in an interview this month that conditions in the mine were worse than anticipated.

“Nobody expected (the acid water backed up in the mine) to be that high,” he said.

The report says, however, that decreased wastewater flows from the mine, which had been leaching for years, could have offered a clue to the pressurization. Also, a June 2014 task order about work at the mine said “conditions may exist that could result in a blowout of the blockages.”

The inability to obtain an actual measurement of the mine water pressure behind the mine’s blocked opening “seems to be a primary issue,” according to the review. It went on to say that if the pressure information had been obtained, other steps could have been considered.

It did not elaborate on what those steps could have been.

Stan Meiburg, EPA’s deputy administrator, said during the call that “provisions for a worst-case scenario were not included in the work plan.”

The review, summarized in an 11-page report and led by five EPA workers from multiple EPA regions and headquarters, includes a list of recommendations for further agency mine work throughout the country, including new approaches to mines at risk of blowout and emergency action plans to deal with such disasters.

The investigative team called the mine’s blowout “inevitable” and said actions by those at the scene — who the review said had “extensive experience” — probably prevented fatalities.

The Department of the Interior is conducting an external review of the spill, and it is expected to be released in October. The Congressional Science, Space and Technology Committee also is investigating the blowout.

Cynthia Coffman, Colorado’s attorney general, on Wednesday said the EPA’s internal report is still being mulled over by her office. She and her counterparts in New Mexico and Utah say they are weighing a lawsuit against the EPA.

The Navajo Nation says it intends to sue.

“A non-federal, independent review is a must,” Coffman said in a statement to The Post. “I don’t trust the EPA or this administration to investigate itself. It would have never allowed BP to investigate the Deepwater Horizon oil spill.”

Jesse Paul: 303-954-1733, jpaul@denverpost.com or twitter.com/JesseAPaul