A California police union said its members might stop working San Francisco 49ers home games after quarterback Colin Kaepernick’s protest of the national anthem over racial injustice and police violence.

Members of the Santa Clara police union asked the 49ers to take action against Kaepernick or “it could result in police officers choosing not to work at your facilities,” according to a letter obtained by NBC Bay Area.

“I’m already hearing it this week that next week on Monday Night Football, some officers are not going to work,” said Frank Saunders, president of the Santa Clara Police Officers Association, according to NBC Bay Area. “The 49ers are allowing this to come out from an employee and it’s making for a hostile work environment for us at the stadium.”

Kaepernick became the focus of national attention last week, when he refused to stand during the national anthem before a preseason game. “I am not going to stand up to show pride in a flag for a country that oppresses Black people and people of color,” Kaepernick said in an interview with NFL Media after the Aug. 26 game. “To me, this is bigger than football and it would be selfish on my part to look the other way. There are bodies in the street and people getting paid leave and getting away with murder.”

Kaepernick received more criticism this week, when he wore socks during practice that featured cartoon pigs in police hats.

Read more: Colin Kaepernick and a Brief History of Protest in Sports

The San Francisco 49ers said Friday that the organization continues to stand by its original statement recognizing Kaepernick’s right to choose whether to participate in the national anthem or not, according to NBC Bay Area.

Kaepernick has defended his protest, responding to critics and telling reporters he is not “anti-American.”

“I realize that men and women of the military go out and sacrifice their lives and put themselves in harm’s way for my freedom of speech and my freedoms in this country and my freedom to take a seat or take a knee, so I have the utmost respect for them,” he said during the 49ers game on Thursday, according to USA Today. He promised to donate $1 million to groups devoted to this cause.

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Write to Katie Reilly at Katie.Reilly@time.com.