the president was quick to tweet about it, suggesting the players can't even explain why they are protesting. here's abc's chief white house correspondent, jonathan karl. >> reporter: it was these new images from the opening of the nfl preseason games that prompted president trump to reignite an old controversy. the nfl players are at it again, trump tweeted. 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 most of them are unable to define. in fact, the players have made it quite clear they are calling for action on issues of racial injustice. that's something the president himself as acknowledged. >> that's what they are protesting. people that they think were unfairly treated by the justice system. >> reporter: philadelphia eagles player malcolm jenkins, who raised his fist in protest, indicated players won't stop. >> where we are now, i just think it's important that we continue to, you know, keep this