Trump administration officials earlier this year killed a federal criminal investigation into the coal empire owned by Sen. Jim Justice, a Republican from West Virginia and a close ally of the president’s.

The investigation examined potential criminal violations of the Clean Water Act by the multistate mining operations largely run by Justice’s son, Jay, according to current and former officials familiar with the matter.

The criminal probe was a significant escalation in the yearslong effort to police serial pollution offenses by Virginia-based Southern Coal and dozens of affiliated mining operations controlled by the family. In the past decade, Southern Coal and other Justice corporations have racked up tens of thousands of alleged violations of the Clean Water Act and have been sued repeatedly by state and federal prosecutors over their failure to properly follow environmental laws at their mining sites.

The investigation shuttered by the Trump administration was a joint effort by prosecutors and investigators with the Environmental Protection Agency, the Department of Justice’s Environmental Crimes Section and the U.S. Attorney’s Office of the Western District of Virginia to probe whether the incessant violations of antipollution laws had risen to the level of criminal behavior, people familiar with the matter said.

People familiar with the investigation told ProPublica that prosecutors believed they had a strong case. They initially had the blessing of Robert Tracci, President Donald Trump’s top official in the Western District of Virginia, to move forward.