The Santa Clara Police Union has threatened to stop working San Francisco 49ers events if the organization doesn't take some sort of action against quarterback Colin Kaepernick, Ian Cull and Brendan Weber of NBC Bay Area report.

Kaepernick has remained seated during the national anthem in protest of racial discrimination and police brutality, particularly against black people. The union is reportedly upset about his comments on police behavior and the fact he wore socks depicting pigs in police uniforms during a practice.

Related: Kaepernick wore pig socks as protest against 'rogue cops'

In the letter, which was obtained by NBC Bay Area, the union doesn't specify what type of action it wants the 49ers to take, but says if something isn't done, "it could result in police officers choosing not to work at your facilities."

The letter adds that the union "has a duty to protect its members and work to make all of their (working) environments free of harassing behavior."

On Monday, the San Francisco Police Officers Association asked the 49ers and the NFL to denounce Kaepernick's protest, calling his comments about police being put on paid leave for murder "ill-advised."