(CBS SF) – Using a mysterious tweet sent out during Super Bowl 50, Seattle Seahawks running back and former Cal star Marshawn Lynch announced he is retiring from the NFL.

During the 4th quarter of Sunday’s game, Lynch posted a picture of cleats hanging from a line, with a peace sign emoji.

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Following Lynch’s tweet, the Seahawks official Twitter account quoted his tweet and added the hashtag #ThankYouBeastMode.

Lynch’s teammates also paid homage.

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On Monday, Lynch’s agent Doug Hendrickson confirmed to The Associated Press that the running back plans to retire.

The 29-year-old has played nine seasons in the NFL, the last six with the Seahawks. Lynch was part of the Seahawks team that won Super Bowl XLVIII.

Prior to going pro, Lynch played three seasons at UC Berkeley, before forgoing his senior year at Cal to enter the 2007 NFL draft. Lynch is a graduate of Oakland Technical High School.

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Tim Fang is a digital producer for CBS San Francisco and a native of the Bay Area. Follow him on Twitter @fangtj.