EPA internal report here.

[T]he Team concludes that the Adit blowout was likely inevitable. Actions taken by the EPA OSC to pull out the site personnel and crew from and near the Adit, just prior to the blowout, probably avoided any fatalities from the pressurized Adit blowout.

Although the removal investigation team was quite experienced and followed standard procedures of a well thought out work plan that included state and ARSG involvement, the underestimation of the water pressure in the Gold King Mine workings is believed to be the most significant factor relating to the blowout.

The Senate Committee on Indian Affairs will hold a hearing about the incident on Wednesday, September 16, 2015.  Chairman Barasso’s (R-Wy) announcement here.