A 10-year plan to clean up Maine’s Androscoggin River, long one of Maine’s dirtiest, is encountering a setback.

The plan grew out of a compromise in the Legislature last spring. Lawmakers agreed to double an interim five-year cleanup goal to ease the $100 million cleanup cost for the paper mills and other river users who contribute to the pollution.

High costs to the paper mills could result in lost jobs, lawmakers reasoned.

Now the 10-year plan, which took a decade to forge, is threatened by a battle over who should pay.

FPL Energy, owner of the Gulf Island Dam between Lewiston and Auburn, is working to overturn the plan and avoid a multimillion-dollar share of the cleanup. FPL says it should be the responsibility of paper mills upriver to pay for the cleanup.

The company has filed an appeal with the state Board of Environmental Protection objecting to the plan and has been laying the groundwork for possible court action.

“It may all fall apart now, which is unfortunate,” Rep. Ted Koffman, D-Bar Harbor, House chairman of the Natural Resources Committee, said of the cleanup.

FPL lawyers revealed a series of closed, undocumented meetings between state officials and a Rumford paper mill on agreements leading to the 10-year plan.

The Department of Environmental Protection later dropped key agreements with two mills, and last week the Attorney General’s Office said it was investigating whether the talks and lack of records violated the state’s Freedom of Access Act.

The river-cleanup plan requires mills in Jay and Rumford and FPL to pump additional oxygen into the river above the dam so fish can swim and reproduce there as required by federal law. Pollution accumulates in Gulf Island Pond above the dam and robs the river of oxygen.

Hundreds of thousands of dollars have already been spent to restore oxygen in the river.