Former San Francisco 49ers quarterback Colin Kaepernick filed a grievance against the NFL on Sunday, alleging that he remains unsigned as a result of collusion by owners following his protests during the national anthem.

Kaepernick started a national conversation about political activism by athletes last season when he decided to sit, and then kneel, during the anthem to bring attention to mistreatment of African-Americans by police. Other players have continued the protests this season, prompting an angry response from President Donald Trump, who said players should be fired for not standing during the anthem.

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