Federal regulators failed to protect workers at Utah's Crandall Canyon mine, where nine people died last summer, a U.S. Labor Department watchdog said Monday.

The department's inspector general's office blamed federal mining regulators for negligence in approving a roof-control plan for the mine. Six miners died in a collapse Aug. 6, and another cave-in 10 days later killed three people trying to reach the trapped men.

The 80-page report Monday from Assistant Inspector General Elliot Lewis said the Mine Safety and Health Administration could not show it made the right decision when it approved risky retreat mining at Crandall Canyon.

In the same report, MSHA director Richard Stickler disputed the claim his agency was negligent and unduly influenced by the mine operator, Ohio-based Murray Energy Corp. MSHA is part of the Labor Department.

"We take exception to the inspector general's use of headline-grabbing language that is unsupported by facts or evidence," MSHA spokesman Matthew Faraci said Monday.

The report placed much of the blame on MSHA's field office in Price, and said agency headquarters exercised little or no oversight.

The field office, it said, ignored Crandall Canyon's active seismic history by failing to check with the University of Utah's Seismograph Stations.

The U.S. Bureau of Land Management, which leases federal land for mining, doesn't have to ensure safety but consistently raised questions about risks at Crandall Canyon, according to the report.

"Thing(s) should get interesting soon," a BLM inspector wrote of cave-in dangers on July 12, less than a month before the disaster.

Retreat mining involves yanking supporting pillars of coal from the mine, allowing the roof to collapse as miners and equipment work their way out.

The bodies of the six miners killed in the first collapse have not been recovered, more than 1,000 below ground.