This is Welles’ radio broadcast from July 28, 1946. It refers to the story of World War II veteran Isaac Woodard. The police officer he addresses was Police Chief Linwood Shull, who was tried in federal court before an all-white jury and found not guilty. The case spurred President Truman to make some changes at the federal level. There are plenty of other links about the event at Metafilter, especially in the comments.