House Republicans are leveling new charges against the Environmental Protection Agency for trying to change the chain of events that led the agency to cause a massive 3 million-gallon spill of toxic sludge in Colorado that sullied the waterways of three states.

A letter from the House Natural Resources Committee sent to the EPA's Office of Inspector General (OIG), obtained by the Washington Examiner, spells out problems with a recent addendum the agency made to its own internal investigation that contradicts official investigations by the Department of Interior and the inspector general's office.

"The addendum states that its new narrative is based on 'a followup interview with the two on-scene coordinators (OSCs) most closely associated with the event,'" the 13-page letter from the committee's Republican leadership reads. "However, the circumstances surrounding this interview raise concerns about its timing, appropriateness and potential to affect the [EPA inspector general's] investigation."

The letter says the interviews were done by a handful of EPA staff with agency coordinators overseeing the cleanup of the Gold King Mine in Silverton, Colo., in August, and there appears to be inconsistencies in how each interview was conducted.

The bigger issue was the timing of the interviews in preparing the addendum, the letter reads. "One of the most concerning problems with EPA's addendum is its timing," the letter says. "According to the addendum, the interview of Hays Griswold and Steven Way occurred on December 2, 2015 — prior to the release of the [inspector general's] report, which is not expected until early 2016."

The letter says the "timing of the interview calls into question the EPA's respect for the [inspector general's] ongoing investigation and commitment to ensuring the integrity of witness testimony."

The EPA said it is reviewing the letter. The Interior Department, which also is investigating the spill, did not respond to requests for comment.

The letter says the EPA's own guidance on investigations "states that 'managers should not question staff about their interactions with the OIG.'" Yet, based on the agency's addendum, "it appears likely that a regional supervisor and two officials from headquarters questioned Mr. Griswold and Mr. Way about matters central to an ongoing OIG investigation, and may have done so following their interactions with the [inspector's office].

"Second, the interview was conducted not by independent investigators or technical experts from unaffected regions, but by three EPA employees with close ties to the agency's public response to the Gold King Mine spill," the letter says.

The letter also says the addendum does not match events as detailed in a recent Department of Interior report that concluded that the toxic spill was caused by the EPA not using a common method of relieving water pressure at abandoned mines to prevent blowouts. The Interior report notes that the method was used by EPA and its contractors at a nearby mine, but not at Gold King.

Instead, the interviews with Mr. Griswold and Mr. Way demonstrated that the EPA applied ample caution before attempting to open the abandoned mine to avoid a spill, the letter says.

"The most alarming aspect of the addendum is that the underlying joint interview of Mr. Griswold and Mr. Way – who have both spoken previously with EPA and [Interior's] Bureau of Reclamation review teams – allegedly uncovered new information that conflicts with the initial EPA Internal Review, the [Interior Department] report, and the work that was actually performed at the site," the letter says.

"The addendum's discovery – four months after the spill – of new information that goes to the crux of EPA's objectives and activities at the Gold King Mine site on August 5, 2015, is indeed startling," the House committee says in the letter.

The "new narrative of events described in the addendum claims that on August 5, 2015, Mr. Griswold was directing the EPA crew in a manner 'completely consistent with the direction provided by [Mr. Way].' The Aaddendum reiterates: 'The work being conducted on August 4 and 5 was completely consistent with the direction provided by the primary OSC [Steven Way] prior to his leaving for vacation, to help plan for the August 14 meeting and potential future work.'"

"This claim is demonstrably false and is one of multiple claims that diverge from the facts and conclusions presented in reports issued previously by EPA and the Interior Department," the letter says.

The letter shows that neither of the previous review teams who interviewed Mr. Griswold and Mr. Way had cited Way's giving verbal instructions "to wait to open the mine" until he returned from vacation on Aug. 14.

"The addendum alleges that Mr. Way provided 'clear verbal direction' to the EPA crew 'not to proceed with any work on actually opening the adit [mine opening] until after his return [from vacation] and the planned consultation on August 14," according to the letter.

"The claim that the EPA crew intended to pause work until August 14 is not supported by the [Interior] report, which states that on August 4 the team 'discussed a plan to open the adit,' [and] discussed the plan to reopen the adit again on August 5, and then immediately 'the contractor began excavating.'"

The EPA addendum says the it is consistent with the Interior Department's report and the EPA's own Aug. 26 internal study. "In summary, the work on August 4 and 5 was assessment and preparatory work being conducted (using EPA removal assessment dollars) to better inform a planned consultation on August 14," the addendum reads. "There was no plan or intent to begin digging out the collapsed/non-engineered plug blockage on August 4 or 5."

The work conducted on August 4 and 5, which was followed by the release of 3 million gallons of contaminated water, "was completely consistent with the direction provided by the primary [lead official] prior to his leaving for vacation, to help plan for the August 14 meeting and potential future work," the EPA said in the report.