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(CNN) —  

President Donald Trump railed against NFL players who protested at Thursday night’s preseason games, saying the players are outraged about “something that most of them are unable to define.”

“The NFL players are at it again - taking a knee when they should be standing proudly for the National Anthem. Numerous players, from different teams, wanted to show their ‘outrage’ at something that most of them are unable to define. They make a fortune doing what they love,” he tweeted from his Bedminster golf club on Friday morning, the ninth day of his vacation.

On Thursday, as the preseason games officially kicked off, several players took a knee, raised a fist, or did not take to the field while the National Anthem was played before a dozen preseason games across the country.

NFL players have said the protests, which began in 2016, are intended to draw attention to what they see as systematic bias against people of color.

“It’s not necessarily about the anthem, that’s where everybody’s messing up,” Tennessee Titans star defensive end Jurrell Casey told CNN last month. “The way that the justice system treats minorities is the issue that we have.”

Trump has previously expressed an understanding of the reasoning behind the protests.

“I am going to ask all of those people to recommend to me – because that’s what they’re protesting – people that they think were unfairly treated by the justice system,” Trump told reporters in June before boarding Marine One. “And I understand that. And I’m going to ask them to recommend to me people that were unfairly treated – friends of theirs or people that they know about – and I’m going to take a look at those applications. And if I find, and my committee finds that they are unfairly treated, then we will pardon them or at least let them out.”

The President sees the issue as a political winner, using the protests to fire up his base. He told attendees of a private dinner last year, “It’s really caught on. It’s really caught on.”

He’s also been criticized recently for questioning the intelligence of several notable African-Americans – including CNN anchor Don Lemon and California Democratic Rep. Maxine Waters.

The tweets come amid rising racial tensions on the anniversary of last year’s violent Charlottesville white supremacist rally where Heather Heyer, a counter-protester, was killed. On Sunday, white nationalists and other right-wing groups are expected to gather outside the White House for another “white civil rights rally.”

In another tweet, Trump also reiterated his belief that players, who don’t stand for the National Anthem should be penalized.

He wrote, “Be happy, be cool! A football game, that fans are paying soooo much money to watch and enjoy, is no place to protest. Most of that money goes to the players anyway. Find another way to protest. Stand proudly for your National Anthem or be Suspended Without Pay!”

NFL players, in fact, do not take home a majority of the revenue from games. According to Sports Illustrated, in the current collective bargaining agreement, players get a lesser share than NFL team owners, calculated around 47 to 48%.

The NFL said it won’t punish the players who took part in Thursday’s protests, noting it had shelved what was supposed to be a new policy on anthem conduct until it reaches an agreement with the NFL Players Association.

Trump has slammed the NFL players since the protests began, suggesting the players be kicked out of the country and praising sports he says are more patriotic.

CNN’s David Close, Jason Hanna, Kevin Dotson, Ray Sanchez, Kaitlan Collins and Veronica Stracqualursi contributed to this report.