Nine months after a deadly coal-mine disaster in West Virginia, federal investigators finally laid out in detail today their working theory for what caused the blast.

They blamed maintenance and equipment failures at the Upper Big Branch mine, but they stopped short of directly accusing mine owner Massey Energy of negligence.

NPR's Howard Berkes reports from Beckley, West Virginia.

HOWARD BERKES: The officials at the Mine Safety and Health Administration said these were not firm conclusions. They still have evidence to analyze, and they want a second round of interviews for some witnesses. But everything they've seen leads them to say again what they first said April 5th.

Mr. KEVIN STRICKLIN (Mine Safety and Health Administration): I mean, we still stand by our point that all explosions are preventable.

BERKES: That's Kevin Stricklin, the nation's coal mine safety chief, who briefed reporters in a teleconference this morning, sending them to their computers to see the slides and videos that bolster that point. We've posted them at

They lay out the scenario an NPR News investigation reported earlier, beginning with a small ignition of methane gas triggered by sparks from the monstrous tool that was cutting coal.

This shearer, as it's called, had multiple maintenance and equipment failures. Some of its carbide-tipped bits were worn down to steel nubbins, which led to more sparks as it hit sandstone in the coal seam. And a water spray system on the device wasn't working.

Mr. STRICKLIN: It's used for dust control, but secondly and most importantly in this case, it's used to quench any frictional ignition that may be occurring.

BERKES: That dust is coal dust, which can feed and magnify a small ignition, turning it into a fiery and concussive force. Investigators found excessive coal dust throughout Upper Big Branch, and they say it wasn't neutralized like it's supposed to be with a substance called rock dust. So when the small ignition erupted, the bigger blast grew as it raced more than two miles underground, taking 29 lives along the way.

Massey Energy's own mine managers didn't report these malfunctioning systems in a required safety inspection before the blast, noted Assistant Secretary of Labor Joe Main.

Mr. JOE MAIN (Assistant Secretary, Department of Labor): The operators have a responsibility to be conducting these examinations to protect the miners. These are things that should have been caught during normal mine operator examinations.

BERKES: These are things that are also supposed to be caught by federal mine safety inspectors, but neither Main nor Stricklin would admit to any failure there. Stricklin says his inspectors wrote hundreds of citations and violations for Upper Big Branch.

Mr. STRICKLIN: In addition, the mining environment changes dramatically in one shift. There's no way that we can say that when we were there last, and the sprays and the bits were in place, that they would have been in place on April the 5th. I think my folks were enforcing the law here.

BERKES: But the agency itself failed to apply the full extent of the law. It failed to invoke a tough administrative procedure for mines with habitual safety violations, and it failed to seek a federal court injunction that could have closed the mine and put it under a judge's supervision. No mine faced that kind of enforcement until after Upper Big Branch exploded.

Massey Energy has its own theory about the blast, which casts it as a natural and unpredictable infusion of gas that was so sudden and vast it overwhelmed all safety systems. Stricklin says all the evidence rejects that.

Mr. STRICKLIN: I can't reiterate this enough: We do not think this was a massive methane explosion. We think it was small and then turned into a coal dust explosion.

BERKES: Massey Energy issued a written statement today saying it does not believe worn shearer bits, malfunctioning water sprayers or coal dust played any role in the explosion. It said it will brief the families of the Upper Big Branch victims soon and then explain its own working theory to reporters.

Federal investigators say they won't have final conclusions for another 60 to 90 days.

Howard Berkes, NPR News, Beckley, West Virginia.

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