BOSTON (CBS) — The NFL and NFLPA still have not settled on a resolution for the national anthem policy, and Thursday night when the preseason began for most NFL teams, two players dropped to a knee during the anthem prior to their games.

Miami Dolphins receivers Kenny Stills and Albert Wilson both took a knee during the national anthem, while several other players around the league either raised a fist during the anthem. Several members of the Jaguars stayed in the tunnel during the anthem instead of standing on the field.

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This news made its way to President Donald Trump, and it didn’t take long for him to send out a couple of tweets on Friday morning that were very clear with their message.

Trump bemoaned the NFL players for being “at it again.” The president said the players are “unable to define” the cause for which they’re promoting, and then threatened that players who do not “stand proudly” for the national anthem would be “suspended without pay.”

It is, of course, not the first time the president has weighed in on the matter. And given the NFL’s lack of resolution on a policy, it likely won’t be the last.

NFL owners pushed through a new policy this offseason which would have punished teams with fines if players demonstrated during the national anthem. Teams would also have been able to punish players. Trump showed support for Cowboys owner Jerry Jones when Jones said that all Cowboys must stand during the anthem.

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That policy was put into place without any input from players or the players’ union, so the NFL agreed to undergo discussions with the NFLPA after the union had filed a grievance. The two sides have yet to reach a solution that works for both sides.

Several NFL players collaborated to write a column in The New York Times in June, in which they explained in a concise 800-word story what they felt to be issues in the criminal justice system. The movement began two summers ago with Colin Kaepernick, who explained at the time that he was taking a stand against the United States, a country which he contended “oppresses black people and people of color.”

The issue largely faded out of the NFL, until Trump last year said at a rally that NFL owners should say “Get that son of a bitch off the field” whenever a player protested. That comment inspired many players to take a knee during the anthem in the week and weeks that followed, and the issue has yet to go away again.

In June, Trump downplayed the concerns of the players, saying this about their cause: “I don’t know if it’s a real issue. I don’t think it’s a real issue.”

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Stills said after Thursday’s protest, “If you continue to misinterpret what we’re doing, reach out to me. Look at my website, look at my Twitter, look at all my social media platforms, and I think you’ll get a better idea of why we are doing what we are doing.”