The Union of Concerned Scientists wants FirstEnergy Corp.'s troubled Davis-Besse nuclear power plant kept shut down until a federal criminal investigation of the plant ends.

The Washington-based advocacy group on Wednesday sent a letter to the Nuclear Regulatory Commission urging the agency to delay any restart until the NRC concludes its criminal investigation. The group has told the U.S. attorney's office in Cleveland it believes there is strong evidence FirstEnergy management committed criminal acts in lying under oath to the NRC.

Davis-Besse may be ready for restart in August.