Members of the Nationals Basketball Association's Boston Celtics and Sacramento Kings came together Sunday to call attention to the killing of an unarmed black man by police.Players wore warmup t-shirts with the message "Accountability. We are one." printed on the front and #StephonClark on the back.Clark died March 18 in South Sacramento. Police suspected him of breaking into cars and when officers confronted him they shot him 20 times believing he was holding a gun. He was later found to have only a cell phone.Sacramento Mayor Darrell Steinberg said he would not second guess "the split-second decisions of our officers" and called for a full investigation of what happened.Protesters have rallied against the shooting during the past week, including one demonstration that blocked fans from entering a Kings game on Thursday. That night, team owner Vivek Ranadive spoke to those who did make it inside the arena with Kings players surrounding him."We recognize that it is not just business as usual, and we are going to work really hard to bring everybody together to make the world a better place," Ranadive said.Sunday's game included a video message with players from both the Kings and the Celtics."These tragedies have to stop," says Kings guard De'Aaron Fox."There must be accountability," says Celtics forward Al Horford.The video goes on to say that "change is necessary" and people need to talk, act and unite. It ends with a plea to "say his name," in reference to Clark.The issue has gained prominence in the past few years with protests that began with National Football League player Colin Kaepernick kneeling during the traditional pre-game playing of the U.S. national anthem to call attention to the treatment of minorities in the country, particularly when it comes to police brutality.Many other NFL players joined him, sparking a backlash from some team owners, fans and President Donald Trump, who said those who knelt should be fired.NBA star Stephen Curry said after Trump's comments in September that he would not make the customary visit championship-winning teams take to the White House.The NBA league office emphasized to teams its own rule that players must stand during the anthem, but also encouraged them to find other ways to connect with their communities. A memo suggested making videos featuring players speaking about issues important to them, having players or coaches make speeches before games, and also participating in community events that help connect people and foster conversations.