SAN FRANCISCO (Reuters) - Kansas City Chiefs cornerback Marcus Peters raised his fist and several Miami Dolphins players knelt during performances of the U.S. national anthem on Sunday, the latest gestures in the National Football League to draw attention to racial inequality.

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San Francisco 49ers backup quarterback Colin Kaepernick started a controversy when he began the protests against injustice and police brutality by refusing to stand for “The Star-Spangled Banner” during preseason games.

Other members of the Chiefs linked arms during the anthem, as did the Seattle Seahawks at their game in Miami, although the exact meaning of their gestures was not immediately clear.

Sunday’s games fell on the 15th anniversary of the Sept. 11 attacks on the United States and many NFL teams commemorated the tragedy with special ceremonies.

Four Miami Dolphins players - Arian Foster, Jelani Jenkins, Michael Thomas, and Kenny Stills - knelt while the national anthem was played at their season opener against the Seahawks. On the other side of the field, the Seahawks locked arms while standing during the song.

Photos published online by Sports Illustrated showed Tennessee Titans players Jurrell Casey, Wesley Woodyard and Jason McCourty raising their fists ahead of their game against the Minnesota Vikings as well. It was not not clear whether the images were taken before, during or after the anthem however.

Representatives for the teams could not be immediately reached.

The gesture on Sunday by Peters, 23, who is African-American, recalled the raised fist demonstration by black athletes Tommie Smith and John Carlos during their medal ceremony at the 1968 Olympic Games in Mexico City.

The other Chiefs players they decided to lock arms as a sign of solidarity after discussing the issue as a group.

“It was our goal to be unified as a team and to be respectful of everyone’s opinions, and the remembrance of 9/11,” the team said in a statement. “It’s our job as professional athletes to make a positive impact on our communities and to be proactive when change is needed.”

Seattle wide receiver Doug Baldwin, in a video he posted online announcing his team’s decision to link arms, said, “Progress can and will be made only if we stand together.”

In Thursday’s NFL season opener, Denver Broncos linebacker Brandon Marshall had knelt during the anthem.

The protesting players have been seen as allies of the Black Lives Matter movement, which grew in response to a string of high-profile police killings of unarmed black people across the country. About two-thirds of NFL players are black.

The protests also have provoked anger in some fans who see the gesture as disrespecting the U.S. flag, the military and the nation in general.

To mark the 15th anniversary of the World Trade Center attacks, the league said it would play videotaped messages from President Barack Obama and his predecessor, George W. Bush, before each game, and a 9/11 decal was to be placed on players’ helmets.

Kaepernick’s 49ers play their opening game against the Los Angeles Rams on Monday evening. Obama has said Kaepernick was exercising a constitutional right and provoking conversation “around some topics that need to be talked about.”