Research shows that police have one trigger finger for whites and another for blacks

A wealth of research and reports exists, not just from recent years but dating to the beginning of modern policing, in the early 20th century, said Tony Platt, a distinguished affiliated scholar at the Center for the Study of Law and Society at the University of California at Berkeley.

Mr. Platt, who has written about the history of policing in America, said racial strife between police departments and racial minorities is well documented.

He cited a congressional committee's report on the bloody East St. Louis riots of 1917, when roughly 3,000 white men marched downtown in the Illinois city and attacked African-Americans after black workers had been hired to replace striking white workers. The committee noted that the police at the riots "shared the lust of the mob for Negro blood."

Though research on police shootings did not begin appearing until the 1960s, previous works detailed the fraught, and violent, relationship between the police and black civilians.

Notably, in 1944, the economist Gunnar Myrdal published An American Dilemma: The Negro Problem and Modern Democracy, in which he portrayed the racial biases of Southern white policemen against the black neighborhoods they patrolled. The book was later cited in the Supreme Court's 1954 decision in Brown v. Board of Education.