Valued exposure: Trooper

A federal trooper stands on watch on a street in Washington D.C. 6 April 1968.

Dozens of major cities in the United States were rocked by an escalation in the race riots following the assassination of Martin Luther King on 4 April 1968.

Several hundred people were injured and about 3,000 people were arrested - most of those in Washington DC.

Photo: Pictorial Parade/Getty Images