Opponents of Maryland’s Purple Line project have filed a third federal lawsuit aimed at stopping the rail line’s construction, arguing that the U.S. Army Corps of Engineers improperly allowed work that will harm streams and wetlands.

The plaintiffs — the nonprofit Friends of the Capital Crescent Trail, and Chevy Chase residents John M. Fitzgerald and Leonard Scensny — alleged in a lawsuit filed Jan. 10 that the corps permit allowing dredging and other work in streams and wetlands is illegal under the Clean Water Act. The agency, they said, should have required the state to demonstrate that it couldn’t improve east-west transit in other ways, such as by supplementing bus service, that would have avoided streams and wetlands. The lawsuit says the corps relied on the Maryland Transit Administration’s analysis and did not independently verify it, as required under federal law.

“They [should] go back to the drawing board and find a project that improves mass transit without messing with streams that are protected,” Fitzgerald, who is also an environmental lawyer, said in an interview.

Sarah Lazo, a spokeswoman for the corps’s Baltimore district, said she couldn’t comment on a pending lawsuit. However, she said, the agency evaluated the construction’s potential permanent impacts to a half-acre of non-tidal wetlands and 5,100 linear feet of streams, including from 40 stream crossings. The state is required to mitigate those impacts by restoring Paint Branch Stream in College Park and creating wetlands in the area of Ken-Gar Palisades Park in Kensington, she said.

Construction on the 16-mile Purple Line started in August 2017, a year behind schedule, after a three-year battle over the first lawsuit filed by Fitzgerald and Friends of the Capital Crescent Trail. The U.S. Court of Appeals for the D.C. Circuit ultimately overruled a lower court judge’s decision that Metro’s declining ridership required Maryland to update the Purple Line’s environmental study.

A second lawsuit, filed in 2017, argues that government officials didn’t properly consider Metro’s financial problems or the Purple Line’s effects on a historical post office and bridge. That lawsuit is pending before a judge in the U.S. District Court for the District of Columbia.

The latest lawsuit is filed in the U.S. District Court for the District of Maryland.

The project’s construction contractor has said those legal delays, along with delays in acquiring right of way along the rail alignment, have added at least $215 million to the project’s cost and almost a year to the line’s scheduled opening date. The contractor says the Purple Line can open in February 2023 at the earliest, assuming work is accelerated. Maryland transit officials have said they are still negotiating with the contractor to open the line in October 2022 but have declined to comment on the delays or any additional costs.