An oil company hasn't paid a $1.5 million fine assessed by North Dakota regulators two years ago for violations of the state's rules for disposing of salt water.
North Dakota regulators touted the fine against Halek Operating as the state's largest civil penalty against an oil company.
The site near Dickinson still hasn't been cleaned up and the company has dissolved.
Court records show federal officials have been investigating the incident for months. U.S. Attorney Timothy Purdon says the criminal investigation is into the company and its owner.
Company owner Jason Halek of Southlake, Texas, also hasn't paid a $26.2 million fine levied in 2010 in a scheme that bilked Texas oil investors.
Environmental groups say North Dakota has done nothing to penalize the company and want more federal oversight.