A coalition of environmental groups has threatened the federal government with a lawsuit over cross-country plans to produce plutonium pits, the cores at the heart of modern nuclear weapons.

In a Tuesday letter to the Department of Energy and its National Nuclear Security Administration, the S.C. Environmental Law Project said additional environmental reviews were needed before officials decide where the key warhead components will be forged for years to come.

“If your agencies continue to refuse to undertake this statutorily required task of preparing a” programmatic environmental impact statement, “we will have no choice but to file the action,” wrote Leslie Lenhardt, a SCELP staff attorney. A related announcement described the legal action as “impending.”