The Mine Safety and Health Agency also could not ensure that its approval of the mining plan at the Utah mine was free of undue influence by the mine's operator, Murray Energy, the Inspector General says in a report issued Monday.
MSHA responded to the report that the word "negligent" was misleading and expressed concern that the independent investigating arm of the Labor Department was implying its decisions were affected by undue influence.
But the Inspector General says its findings "remain unchanged."
The IG did not probe the cause of the disaster that claimed six miners -- still entombed some 1,900 feet in the mine -- and killed three would-be rescuers in August. The investigation is one of several ongoing into the tragedy, including a review by MSHA's accident team expected to be completed in the next few months.
Murray Energy did immediately respond to a request for comment.
A report recently published by the Senate Health, Education, Labor and Pensions Committee called for the Justice Department to launch a criminal probe into actions by Murray Energy surrounding the Crandall Canyon disaster. Murray Energy called that suggestion partisan grandstanding.