Miami Dolphins receivers Kenny Stills and Albert Wilson kneeled during "The Star-Spangled Banner" and defensive end Robert Quinn raised a fist during the song ahead of the team's season opener against visiting Tennessee.
Several NFL players kneel during the singing of the national anthem before games to protest police brutality and social injustice.
Wow, NFL first game ratings are way down over an already really bad past year comparison.
Both sides agree that they should avoid saying or doing anything to further stoke emotions about that interest or bring more attention to it, and in the preseason there were fewer than five players protesting in some manner each week, though obviously that could change in Week 1 of the regular season.
"They have not backed down, even when attacked and intimidated", Kaepernick said in the tweet, which was accompanied by a picture of the Dolphins kneeling before their game.
"Wow, NFL first game ratings are way down over an already really bad previous year comparison", the president tweeted, asking if ratings would improve "if the players stood proudly for our Flag and Anthem".More news: US Rapper Mac Miller, 26, Found Dead At Home, Drug Overdose Suspected
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ASports crew at the game witnessed the three players' demonstration during the anthem.
Kaepernick became a free agent after facing backlash for kneeling in a sign of protest against racial inequality during the 2016 National Football League season.
Hours before Kaepernick submitted his Thank You tweet, Donald Trump jumped onto the platform to remind folks of his stance.
No players protested during the national anthem before the Patriots faced the Houston Texans.
The NFL remains without a decision or an official stand or policy regarding taking a knee during the anthem - and that is how it will stay, according to Newsweek, citing an ESPN report. "We're trying to move past the rhetoric of what's right or what's wrong in terms of the anthem, and really focus on the systematic issues that are plaguing our communities".