madrid u.s. president barack obama warn attack police racial bias would hurt anti-racism black live matter movement days sniper kill five police officer dallas apparent revenge police shoot black people obama say although activists black live matter movement want see good relations communities law enforcement violence overly broad criticism police undermine protest movement want say say everyone concern ... racial bias criminal justice system maintain truthful serious respectful tone go help mobilize american society bring real change obama say whenever us concern failures criminal justice system attack police disservice cause tell news conference madrid one-day visit micah johnson black u.s. military veteran afghan war open fire police officer thursday protest dallas fatal shoot two black men police louisiana minnesota previous day johnson want kill white people especially white officer dallas police chief david brown say attack thursday night black live matter group say friday advocate dignity murder response shoot also leave seven police officer two civilians wound obama say legitimate criticism make criminal justice system citizens continue protest would hope police organizations also respectful frustrations people communities feel dismiss protest political correctness politics attack police say obama say attack police hurt black live matter cause