WASHINGTON — The federal agency responsible for mine safety failed to carry out required inspections at 15 percent of the nation's underground coal mines, according to an internal Labor Department report.

The report, by the department's inspector general, also said Mine Safety and Health Administration records of an inspection of the Crandall Canyon mine in Utah, where six miners died in roof collapse in August, were dated four months before the inspection started.

The records covered "a requirement for the inspector to evaluate the roof control plan," said the report, which was obtained by The Associated Press on Friday. "The inspector could not explain why the forms were dated before the inspection period."

According to the inspector general's report, the mine safety agency's Office of Coal Mine and Health "did not perform all required inspections at 107, or 15 percent, of the nation's 731 underground coal mines in fiscal year 2006."

The report blamed the missed inspections on "decreasing inspection resources" and management "not placing adequate emphasis on ensuring the inspections were completed."

Richard Stickler, Labor's assistant secretary for mine safety and health, in a response to the report, challenged its numbers.

"Your final report fails to acknowledge that the majority (70 percent) of incomplete mandatory inspections determined during your audit period were at mines that were either non-producing, inactive, intermittent or abandoned during the inspection period," Stickler wrote in a letter to the inspector general. "For inspections not completed at inactive or abandoned mines, miners were not placed at risk to hazardous conditions."

The United States has had three major fatal accidents in underground coal mines in the last two years.

Most recently, six miners were trapped 1,800 feet below the surface of the Crandall Canyon on Aug. 6. It was never learned if they survived the initial cave-in and their bodies have yet to be recovered. Three others were killed as rescuers tried to tunnel toward the trapped miners.

"Missed or incomplete inspections place miners at risk because hazardous conditions in the mines may not be identified and corrected," the report said.

At least one or more statutorily required inspections were missed at the 107 mines, the report said. A total of 147 total required inspections were not completed, the report said.

Also, the inspector general said the Mine Safety and Health Administration could not provide adequate assurance that critical inspection activities were performed. "Our review of 21 inspections of active mines disclosed that for the 68 selected inspection activities we tested, 15 percent were not documented as having been performed because management did not require inspectors to document all critical inspection activities performed," the report said.

Stickler, in his response, said his inspectors should spend their time "identifying and abating hazards as a result of inspections rather than documentation and paperwork."

The mine safety agency has hired and is training additional mine inspectors and is rotating them into understaffed districts to help complete all inspections, the report said.

A Senate subcommittee on Friday subpoenaed the owner of the Crandall Canyon mine, Robert E. Murray, CEO of Murray Energy to testify on Dec. 4 about the accident.

"We're going to get to the bottom of what went on there," said Sen. Arlen Specter, ranking Republican on the Appropriations' labor subcommittee.

"Murray is an indispensable witness, and, candidly, he really flouted the responsibility and authority of the United States Senate to have his testimony to find out what happened so we could do our utmost to prevent future occurrences," Specter said.