Why US soccer star Megan Rapinoe doesn't sing the national anthem

As the national anthem played before the U.S. women's national team took on Thailand in the FIFA Women's World Cup last week, the team stood in a row on the field.

A camera panned down the line of players during the broadcast, showing each with her hand over her heart, mouthing along to the song.

At the end of the line was Megan Rapinoe, the co-captain and one of the biggest stars of the team. She stood impassively, hands at her sides, not singing along.

Rapinoe's silent observance of the anthem is part of a years-long protest by the athlete.

In September 2016, Rapinoe was one of the first athletes to follow then-NFL quarterback Colin Kaepernick's lead in kneeling during the anthem to protest racial oppression and police brutality against black people.

"Being a gay American, I know what it means to look at the flag and not have it protect all of your liberties. It was something small that I could do and something that I plan to keep doing in the future and hopefully spark some meaningful conversation around it," she said at the time, according to The Associated Press. "It’s important to have white people stand in support of people of color on this. We don’t need to be the leading voice, of course, but standing in support of them is something that’s really powerful."

She further explained her demonstration in an October 2016 piece for The Players' Tribune, where she cited "over-policing, racial profiling, [and] police brutality."

U.S. Soccer, the governing body of the sport and national team, while not naming her, did not support her kneeling, according to reports at the time. "As part of the privilege to represent your country, we have an expectation that our players and coaches will stand and honor our flag while the National Anthem is played," the group said in a September 2016 statement, according to a tweet from sports reporter John D. Halloran.

Months later, U.S. Soccer added a policy requiring players to "stand respectfully during the playing of the national anthems at any event in which the Federation is represented." U.S. Soccer declined to comment to ABC News.

Since then, Rapinoe has followed that rule, standing during the anthem, but she regularly does not put her hand to her heart nor does she sing along like her teammates.

"I'll probably never put my hand over my heart," she told Yahoo! Sports in May. "I'll probably never sing the national anthem again."

She told Yahoo! Sports she's still driven by inequality and injustice and added that she believes under the presidency of Donald Trump -- who she called "sexist," "racist" and "not a good person" -- she is "a walking protest." (Trump for his part has said he's the "least racist" person and insisted that he respects women.)

In the Yahoo Sports interview, Rapinoe did not back down from criticizing U.S. Soccer, who she is also suing -- along with her 2015 World Cup teammates -- for gender discrimination, which the organization has denied.

She called U.S. Soccer's references to patriotism to stop her protest "pretty cowardly," likening it to the NFL.

"We can actually have a conversation, instead of just telling me that it's a privilege to pull on the jersey," Rapinoe added. "Like, of course it's a privilege for me to pull on the jersey. Part of that privilege is representing America, and representing America is representing all of America."