4 off-duty officers from the Minneapolis Police Department were working security for the WNBA Lynx game when they abandoned their posts. The four saw the players’ pre-game shirts, became upset, and walked off.

The players were wearing black T-shirts that had the names of two black men who had been shot by police. One name was Philando Castile, a man shot and killed by police during a traffic stop in Minnesota.

The back of the shirt was also printed, and featured a Dallas police badge. This was in honor of the five Dallas police officers killed on the job. The back of the shirt also had the words “Black Lives Matter.”

During the pre-game conference, the players explained that the shirts were made to honor and mourn the dead, and also to advocate for change.