The Trump administration has filed far fewer civil cases accusing companies of violating environmental rules than its predecessors, an analysis of federal data shows.

In the first full six months of Mr. Trump’s second term, the Justice Department filed 11 civil lawsuits against major polluters for breaking bedrock environmental laws, compared with 30 of these cases in President Joseph R. Biden Jr.’s first full six months in office, according to the analysis by the Environmental Integrity Project, an advocacy group.

In addition, the Trump administration reached 18 settlements in civil cases against major polluters, compared with 53 settlements in the same period under Mr. Biden, the analysis found. Such settlements often require companies to pay millions of dollars in penalties.

The slowdown in enforcement has let some polluters off the hook and left communities more exposed to hazardous pollution, former officials, environmental lawyers and activists say.

“The enforcement of environmental laws has fallen off a cliff during the Trump administration, and that reluctance to hold big polluters accountable in court poses a dangerous threat to the health and well-being of the American people,” said Jen Duggan, the executive director of the Environmental Integrity Project.