Staff scientists at the U.S. Environmental Protection Agency had serious concerns last year that the PolyMet copper-nickel mine might not be able to meet state and federal water quality standards under the Clean Water Act, according to documents released Thursday.

The scientists' comments were written more than a year ago in response to a key permit the Minnesota Pollution Control Agency issued to PolyMet Mining. But they were never submitted. Instead, they were released this week as part of a Freedom of Information Act case an environmental group filed against the EPA in an attempt to make them public.

In court filings, the St. Paul-based advocacy group WaterLegacy questioned whether the EPA comments had been suppressed as the MPCA developed the final water quality permit it eventually granted to PolyMet last December.