A group that includes activist investor Jana Partners and NFL player Travis Kelce says it has accumulated one of the largest ownership stakes in Six Flags Entertainment and intends to press the company's leadership on ways to improve the struggling amusement park operator's business.

Jana said Tuesday that the investor group now owns an economic interest of approximately 9% in Six Flags. The group plans to "engage" with Six Flags' management and board of directors to discuss ways to enhance shareholder value and improve visitors' experience.

Shares in the Charlotte, North Carolina-based Six Flags surged 17.7% on the news. The shares added another 5.1% gain in after-hours trading. Even with Tuesday's rally, the company's shares are down about 47% so far this year.

Six Flags reported a

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