The Mine Safety and Health Administration has never developed a reliable system for identifying mines with safety problems despite more than three decades of trying, an agency watchdog said Wednesday.

The Labor Department's inspector general blamed the failure on a 32-year "lack of leadership and priority" over six presidential administrations and a process that fell victim to competing interests of the mining industry and labor unions.

Inspector general Elliot Lewis conducted the report in the wake of the April explosion that killed 29 men at West Virginia's Upper Big Branch mine. The mine had a lengthy history of safety violations, but a computer error in MSHA's screening process for detecting the most dangerous mines allowed it to evade heightened scrutiny.

The report comes two days after mine safety officials announced new screening criteria to crack down on mines with a history of safety problems. MSHA director Joe Main called the new process a "stopgap measure" until Congress passes legislative reforms.

Democrats in Congress said the latest report is more evidence of the need for comprehensive mine safety legislation to revamp a broken system.

"Without meaningful legislative reforms, miners will remain in the crosshairs of reckless mine owners operating outside of the margins of safety," said California Rep. George Miller, chairman of the House Education and Labor Committee.

The report said the computer screening system used since 2007 to identify mines with a pattern of safety violations was "complex and unreliable." Tests showed the system contained errors that sometimes left out mines that should have been flagged, and other times incorrectly included mines that were safe.

Lewis' report faulted MSHA's failure to monitor plans submitted by mine operators to improve safety once they were notified of possible sanctions. He also found that delays in testing rock dust samples could create delays in identifying safety hazards.

The report called on MSHA to create a better system for rooting out bad actors in the mining industry, with criteria and procedures "that are more transparent and well reasoned."

In July, a House committee approved sweeping legislation that makes it easier for the government to shut down problem mines. But the mine industry has objected to the bill as overreaching and premature until the cause of the Upper Big Branch accident is determined. Republicans also have concerns because the bill would overhaul job safety laws affecting nearly every private business in the country.