The April coal mine explosion at the Massey Energy Co.’s Upper Big Branch (W.Va.) mine that killed 29 miners “was preventable” if the mine had been in compliance with federal safety rules, Mine Safety and Health Administration (MSHA) officials told the families of the victims last night.

MSHA officials briefed the families on its findings in a closed-door meeting, but family members who spoke with reporters later said MSHA coal administrator Kevin Strickland said the blast could have been prevented if a coal cutting machine had been properly maintained and if highly explosive coal dust had been controlled.

Today in a briefing with reporters, MSHA detailed what it believes was the cause of the blast.  On the day of the blast, a long-wall mining machine was cutting coal about two miles underground in a mine that was known to have high levels of flammable methane gas.

The cutting head of the machine consists of hundreds of carbide-tipped steel teeth that rip the coal from the seam. The carbide prevents sparks that could occur if the bare metal strikes rock. In addition, the cutting head is equipped with water sprayers that also reduce chances of sparking and help control coal dust. MSHA officials believe coal dust had been allowed to build up in the mine because of improper rock dusting that normally is used reduce the concentration of coal dust in the atmosphere.

But, according to the MSHA investigation, some of the carbide tips had been worn down to bare metal and the water sprayers were not working properly. At the same time, methane had seeped into the area where the mining was taking place.

NPR’s Howard Berkes writes:

So, when methane hit the sparking shearer, a small ignition began. An MSHA official told the crowd that these small methane ignitions are common, occurring somewhere underground as much as once a week, but they rarely evolve into massive explosions.

But at Upper Big Branch, without working water sprayers, the investigators said, the small methane ignition persisted. Floating coal dust fueled it, and when it finally blew, the resulting blast was fed by coal dust spread throughout the mine, which explains an explosion that turned corners and killed along a two-mile path.

In a statement this afternoon, Massey said it did not believe the cutting bits or water sprayers played a role in the explosion and clamed that coal dust was properly maintained.

MSHA’s final report is due in two to three months. Federal prosecutors continue a separate criminal investigation.