The retirement system contends NorthWestern's board is not listening in good faith to offers to buy the Sioux Falls-based corporation, which provides electricity and natural gas to about 600,000 customers in South Dakota, Montana and Nebraska.

The company announced late last month that it had entered into confidentiality agreements with "a select number" of possible suitors, but did not name them. Rapid City-based Black Hills Corp. and Montana Public Power, a coalition of Montana cities, are among those that have expressed interest in buying the company. The NorthWestern board has implemented a stockholder rights plan -- a so-called "poison pill" -- that's meant to discourage takeover attempts. A federal judge earlier denied the plaintiffs' request for a temporary order to suspend the stockholder rights plan.

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