Stepped-up enforcement of mine safety laws has caused such a backlog of new cases that lawmakers warn the delays could hinder a crackdown on repeat violators.

Regulators who handle mine safety enforcement cases tell a House panel the backlog has jumped from roughly 2,700 cases in 2006 to about 16,000 cases now.

The spike came after Congress passed tougher mine safety legislation in 2006 after West Virginia's Sago mine disaster.

Some Democrats claim mine operators are purposely clogging the system by contesting more violations. The mining industry blames new regulations for eliminating more informal procedures to compromise penalties.

Federal officials say they could start to trim the backlog in about three years by streamlining procedures and hiring more staff.