The Nuclear Regulatory Commission is investigating whether wrongdoing occurred in the handling of a leaking cooling tank at the Palisades nuclear power plant in southwestern Michigan.
The Kalamazoo Gazette said the NRC's Office of Investigations is looking into the matter.
Spokeswoman Victoria Mitlyng said on Thursday she can't provide further details.
Entergy Energy owns the plant in Van Buren County's Covert Township.
Spokesman Mark Savage declined to comment on the investigation.
The company shut down Palisades June 12, to fix the leaky tank.
Savage said the work continues.
The NRC also has asked Entergy for a copy of a report the company is developing about the safety culture at Palisades, which the agency has designated as one of the nation's four worst-performing nuclear plants.
Savage says Entergy will comply with the request.