WASHINGTON — More than a year has passed since the U.S. Department of Veterans Affairs admitted the cost of a new hospital in Aurora had exceeded its budget by more than $1 billion.

But even now, it’s hard to get an official explanation from the VA or its internal watchdog on why the hospital’s cost skyrocketed from about $604 million in 2011 to about $1.7 billion in 2015.

The VA is refusing to make public its own review of what went wrong. And a second investigation by the VA Office of Inspector General — once set to be released in mid-June — now is not expected until after Labor Day.

“The release of the audit on the Aurora, CO hospital is delayed because OIG has new review processes. At this time we anticipate the audit results being available to the public in the fall,” wrote Michael Nacincik, a spokesman for the VA Inspector General, in response Monday to questions about the report’s release.

The VA Inspector General acts as the agency’s independent auditor and investigator.

Both dead-ends have frustrated members of Colorado’s congressional delegation, who spent much of 2015 trying to convince skeptical colleagues to fund the facility in the face of widespread anger at the veterans agency.

U.S. Rep. Mike Coffman, R-Aurora, has been one of the VA’s most vocal critics.

Last week, Coffman sent a letter to VA Secretary Robert McDonald asking that he make public the review the VA already has completed — as well as the supporting documents that led its investigators to conclude in March that no new punishments were necessary for the cost overrun.

“I urge you, again and hopefully for the last time, to release the complete Administrative Investigation Board (AIB) report on the Aurora, Colorado Replacement Medical Center construction project to all members of Congress and the American people,” wrote Coffman in a letter dated July 7.

Coffman noted the VA had released a summary of its findings to Congress in March, but in doing so omitted 71 exhibits that comprised “thousands of pages, on which the AIB’s conclusions were based.”

Now he wants those, too.

“Congress and the public always have a right to know how tax dollars are being spent,” he wrote. “But especially in this case, given the gross magnitude of waste and mismanagement, transparency is crucial.”

Asked about the reports, a VA spokesman said the agency welcomed Coffman’s interest but then pointed to a previous statement by VA Deputy Secretary Sloan Gibson as to why the VA didn’t want to release the AIB investigation.

“As soon as you start making the material from those AIBs public, you wind up killing the whole investigative process,” Gibson said. “So by doing that, we wind up jeopardizing the future of that device as a means of getting at the ground truth, and I’m not going to do that.”

An investigation by The Denver Post last year traced the hospital’s cost overrun to widespread VA mismanagement and poor federal oversight.

Denver Post reporter John Frank contributed to this report.