American swimmer Anthony Ervin has joined the growing number of athletes to kneel during the national anthem, taking a knee Sunday after he anchored Team USA’s mixed 200-meter medley relay team during a meet in Brazil.

Ervin tweeted out what appears to be an explanation on Monday morning:

As noted by Swim Swam, Ervin’s father is a black Vietnam veteran while his white mother is of Jewish descent. At the 2000 Summer Games in Sydney, Ervin became the first U.S. swimmer of African American descent to win Olympic swimming gold. Sixteen years later in Rio, he became the oldest swimmer in history to win an Olympic gold medal, winning the 50 freestyle at the age of 35.

Anthony Ervin of USA celebrates with his gold medal in the 50-meter freestyle at the 2016 Rio Olympics last December. Reuters/Marcos Brindicci

A sizable number of NFL and WNBA players have protested police brutality and other issues during the playing of the national anthem, but the practice has been slow to spread to other sports. In September, Bruce Maxwell of the Oakland Athletics became the first Major League Baseball player to kneel during the anthem, and on Saturday a German professional soccer team took a knee during pregame ceremonies (no anthem was played) as a show of solidarity with the protesting U.S. athletes.

The next stage could be at the Winter Olympics in PyeongChang, South Korea, which begin in February. The Olympic charter forbids political demonstrations inside the venues, but that might not be enough to stop athletes from protesting. At the 1968 Mexico City Games, U.S. track athletes Tommie Smith and John Carlos memorably raised their fists in a Black Power salute while on the medal stand.

“I am an African-American woman and my family is all African American,” Kehri Jones, a U.S. Olympic bobsled hopeful, told the New York Times last month. “I worry about my family and my siblings and my father going out and ending up in some of these awful situations,” she said, referring to police brutality.

“It’s a really fine line I’m walking because my dad is in the military and I want to represent our country well,” Jones added.”But I also want to represent all the social injustice that has been going on.”