While standing by its earlier report on the April 2010 explosion at the Upper Big Branch Mine, an internal review of the U.S. Mine Safety and Health Administration (MSHA) found that inspectors either missed problems or failed to inspect the areas where they existed in the 18 months before the explosion that killed 29 miners. However, the internal review concludes there’s no evidence those failures caused the disaster.

In MSHA’s report of the accident the agency said that Massey Energy made “systematic, intentional and aggressive efforts” to hide problems and throw off inspectors, even falsifying safety records. Managers also alerted miners when inspectors arrived, allowing time to disguise or temporarily fix dangerous conditions.

Since then, the agency conducted a review of its own actions prior to the explosion and acknowledges multiple failures by field staff in MSHA’s largest region, southern West Virginia's District 4. It also said their effectiveness was compromised by internal communication problems and by federal budget cuts that had created staffing shortages, inexperience and a lack of sufficient training and managerial oversight, the Associated Press reported.

MSHA director Joe Main said he takes the findings seriously and praised the review team for identifying systemic breakdowns. “We can’t just do internal reviews. We have to fix the problems,” Main said. “We take responsibility for the agency's actions here. We have an obligation to fix these things, and yes, we're going to do that.”

While the report focuses on systemic failures, Main said he will review whether administrative actions should also be taken against individuals. But he said blame for the disaster continues to rest squarely with former mine owner, Massey Energy which was bought last summer by  Alpha Natural Resources.

Four investigations have concluded the blast was sparked by worn and broken equipment, fueled by a deadly buildup of methane and coal dust, and allowed to spread because of clogged and broken water sprayers.

The former superintendent of the mine has been charged with conspiracy to commit fraud and is cooperating in a Department of Justice investigation. A former security chief, meanwhile, has been sentenced to three years in prison for lying to investigators and attempting to destroy records.

The internal review said MSHA inspectors consistently failed to identify problems with accumulations of explosive coal dust and deviations from ventilation and roof control plans. It also said they failed to use the operator's examination books to determine whether hazards had been corrected.

It noted those inspectors failed to identify 10 safety violations that MSHA’s accident investigation team later determined had contributed to the blast. In some cases, they didn’t recognize hazards, the report said. In others, they just didn’t inspect the areas where they existed.

Although inspectors wrote a total of 684 violations in the 18 months before the blast, the report said they failed to act on eight that could have been deemed “flagrant,” the most serious designation. They also failed to conduct special investigations on at least six occasions to determine whether managers knowingly violated safety standards.

The report, conducted by MSHA employees outside District 4, found that "inadequate direction training and supervision" was as much a problem as inexperienced inspectors.