The operator of a collapsed Utah coal mine violated safety protocols by cutting vertical seams of coal that should have been left standing as pillars to prevent cave-ins, U.S. government mining regulators said Thursday.

Six miners died last August in the collapse at the Crandall Canyon mine in Price, Utah, southeast of Salt Lake City. Three rescuers were killed by a cave-in ten days later.

A report released by the Mine Safety and Health Administration [MSHA] says a subsidiary of Ohio-based Murray Energy Corp. undermined other pillars by excavating coal from tunnel floors.

The disaster was caused by poor planning, bad design and decisions not to adapt to conditions inside the mine, MHSA chief Richard Stickler said.

"It was not, and I'll repeat, not a naturally occurring earthquake but, in fact, a catastrophic outburst of the coal pillars," Stickler told a news conference on Thursday.

Company president Bob Murray has insisted that the collapse was caused by an earthquake.

The administration said Murray Energy misled regulators about the dangers and violated its approved mining plan.

The company says it is preparing a response.

The first details of the administration's report come from a summary issued to reporters and interviews with family members and one of their lawyers.

The bodies of the six men killed in the initial collapse have never been recovered. Their families launched a lawsuit against Murray Energy in April, alleging negligence.

An attorney for Murray Energy called the allegations "blatantly false and unnecessarily hurtful," and said they were an attempt to smear Murray.

Murray Energy, which purchased Crandall Canyon in August 2006, shares ownership with the Los Angeles Department of Water and Power and the Intermountain Power Agency, a utility consortium comprising two dozen Utah municipalities.

With files from the Associated Press