WASHINGTON (Reuters)—Federal investigators said Wednesday a small fire caused by a methane or natural gas leak likely set off a massive blast of coal dust that killed 29 miners at Massey Energy's coal mine last year.

Several sprinklers that might have quashed the initial explosion were not functioning at Massey's Upper Big Branch mine, according to a preliminary assessment by officials at the Mine Safety and Health Administration.

The teeth on the longwall shearer used to cut coal and stone in the mine were worn down, showering the area with sparks that may have ignited the gas leak, according to Kevin Stricklin, a coal administrator at MSHA.

"There was an ignition...a burst of flame, something that went on for 70 to 90 seconds before there was a mass explosion," Mr. Stricklin told a conference call with journalists.

The bodies of two miners who were believed to be working near the shearer were found 400 to 500 feet away, indicating they had been running from the site when the large explosion occurred, he said.

The April 5, 2010, explosion at the Upper Big Branch Mine in West Virginia was the deadliest mine accident in the United States in 40 years and prompted the resignation of the company's combative CEO, Don Blankenship, in December.

Massey said it did not believe there were issues with the shearer's bits or water sprinklers and did not believe that coal dust played a meaningful role in the blast.

"We currently believe the mine was well rock dusted and that the mine exploded due to an infusion of high levels of natural gas," Shane Harvey, Massey's general counsel, said in a statement.

Despite the potential safety violations cited in the call, MSHA officials defended the agency's oversight of the mine and said Upper Big Branch was the most-cited mine for infractions in the nation.

MSHA expects to conduct follow-up interviews as part of its investigation into the blast, and will likely issue a report on the cause in the next two to three months.

The U.S. Department of Justice and state of West Virginia are also conducting their own investigations.