As was the case every Halloween, on October 31, 1989, a couple of Washington Post reporters were sent to cover the festivities in Georgetown, where thousands of mostly white and affluent revelers partied through the night. I was dispatched to the other side of town—Potomac Gardens, a rundown public housing project a mile east of the Capitol, where Mayor Marion Barry was scheduled to make nice with the residents. My job was to take notes and contribute a few paragraphs to an innocuous story about how Washingtonians celebrated the holiday. I had already been to Potomac Gardens a handful of...