Buyout firm MidOcean Partners and theme park operator Herschend Family Entertainment Corp. offered less than $650 million for the six locations, said the people, who declined to be identified because the process is private. Real estate investor CNL Financial Group offered at least $650 million, one of the people said.

At least one of the suitors — Atlanta-based Herschend — did not include Magic Mountain in its bid.

"We continue to do research on the selected Six Flags properties. Magic Mountain is not specifically involved in our negotiations," said Lisa Rau, spokeswoman for Herschend.

Herschend's exclusion of the roller-coaster-intensive park is likely to hurt its bid because Six Flags seems intent on selling the six properties as a group, said David Miller, an analyst at Sanders Morris Harris in Los Angeles.

Six Flags unveiled plans to sell the parks in June. Bids were due November 7.

The theme park operator may need $800 million to make their debt load manageable.