Federal investigators searching for the cause of the Upper Big Branch mine explosion in West Virginia that killed 29 miners last year believe they now know what touched off the blast: a badly broken cutting tool gave off sparks that, in turn, ignited methane gas.

The massive cutting tool, known as a shearer, was producing more sparks than usual, U.S. Mine Safety and Health Administration investigators explained Tuesday at a closed-door briefing for relatives of the victims. The Charleston Gazette reports that MSHA officials believe the shearer was in a “horrible state of disrepair — with worn out bits and missing or malfunctioning water sprays intended to keep down dust and sparks.”

The Gazette, citing several people who attended the meeting in Beckley, W. Va., said MSHA chief Joe Main told the gathering: “The shearer was totally out of compliance.”

NPR, which also spoke to participants in the meeting, reports that the investigators were careful to say that they have not reached final conclusions and noted that their final report is still 60 to 90 days away. They also did not pinpoint the source of the methane gas. “But they presented specialist after specialist who discussed detailed evidence,” NPR says.

At the time of the explosion, a mining machine was working a coal seam deep inside the mine. Methane had seeped into the “tailgate” area of the machine, investigators said, and was ignited by sparks from the shearer, which were worse than usual because some of the tool’s carbide-tipped teeth were worn down to bare steel. Without working water sprayers, the small ignition persisted and, fueled by coal dust, eventually erupted into an explosion that turned corners and killed along a two-mile path — leading to the nation’s worst mining disaster in 40 years.

“The combination of sparks, coal dust and methane, and no water, formed a volatile mix,” NPR said.

Kevin Stricklin, MSHA’s administrator for coal, reportedly told the families Tuesday that with any number of safety measures — working water sprays or reducing the amount of rock dust, for example — the small methane ignition could have been extinguished in 10 to 15 seconds. “This was preventable,” he said.

MSHA’s working theory conflicts with the view of the mine’s owner, Massey Energy, which has insisted that the blast was caused and fueled by an unpredictable and natural infusion of methane or natural gas from a crack in the mine floor. The company’s general counsel issued a statement Wednesday that Massey did not believe problems with the shearer contributed to the explosion, and that the mine was “well rock dusted.”

Massey is scheduled to brief victims’ families about its own investigation on Friday.