The Labor Department's inspector general says federal mine safety officials have not been excessively enforcing the law in South Dakota.
U.S. Sen. John Thune requested the investigation in 2011 in response to complaints from South Dakota gravel pit operators that they were being regulated as if they were running deep coal mines.
The inspector general's office says enforcement by the Mine Safety and Health Administration increased across the board in the 2009 fiscal year due to a new law that mandated higher inspection levels and the hiring of 170 new inspectors.
The report says that enforcement numbers were higher in South Dakota than the national average, but the jump was traced to two mine operators whose penalties were resolved on appeal.
The inspector general is making no recommendations.