Though shuttered for a year, a Western Maryland paper mill's byproducts continue to pollute the Potomac River, Maryland Attorney General Brian Frosh charged in a federal lawsuit.

The suit filed Thursday, the latest in a battle against the Luke Mill in Allegany County, stems from a fisherman reporting in April 2019 that a black liquid was seeping into the North Branch of the river from the area of the plant.

Tests conducted by both Verso Corp., which owns the mill, and the Maryland Department of the Environment, found the seepage contained levels of mercury, lead and other chemicals that exceeded allowable limits, according to the suit.

The discharge appeared to contain "pulping liquors" that result from the paper-making process and are considered "caustic and corrosive" and can cause burns and respiratory problems.

In December, Frosh sued the company in Allegany County Circuit Court over what he alleged were multiple violations of state environmental laws.

He filed Thursday in U.S. District Court in Baltimore in conjunction with an effort to intervene in a lawsuit filed in March by the Potomac Riverkeepers on similar issues.

A Verso spokesperson responded Thursday evening with a written statement.

"We are reviewing this filing, which makes claims nearly identical to claims that were made in Maryland state court in December 2019, and which Verso has been actively working to address," it said.

"Verso has been working in cooperation for nearly a year and has a remedial plan approved with state regulators from both Maryland and West Virginia, to take immediate action regarding the Luke Mill closure and impact to the North Branch of the Potomac River to remedy these issues," according to the statement.

The federal suit says Verso had stored the pulping liquors in tanks on the West Virginia side of the Potomac but, under orders from that state to empty them, had piped the material to containers on the Maryland side.

Verso has been under orders by the Maryland Department of the Environment for more than a year to identify where the leakage is coming from and to contain and remove it, the lawsuit said. But the plan Verso submitted does not identify steps that would stop the discharge permanently and remediate the contaminated area, according to the suit.

Additionally, MDE had directed Verso to erect signs in the area of the seepage warning of the hazardous material and instructing people not to drink or come in contact with the water, according to the lawsuit. Instead, the signs merely indicate the area is restricted and do not use MDE's stronger warning, according to the suit.

"Verso continues to contaminate the Potomac River with toxic pulping liquor," Frosh said in a statement. He called it "devastating for water quality and harmful to fish and wildlife."

Frosh's office said the case in state court remains pending, but would be dismissed once the intervention in federal court is finalized.

The federal case adds additional counts of endangerment under federal conservation law, and seeks reimbursement for the state's costs for addressing the seepage. The attorney general's office did not offer an estimate of those costs, but said they are ongoing.

Ben Grumbles, Maryland secretary of the environment, said in a statement that the state is "fighting in federal court for the health of our Potomac River and the vitality of our communities," and that he will push for a "productive reuse" of the mill site.

Verso closed the 131-year-old mill in May 2019, and nearly 700 people lost their jobs.