Beekman Winthrop, an independently wealthy descendant of a prominent New England family, whose ancestors included the first governor of the Massachusetts Bay Colony, was a freelance investigative reporter in the early 1970s, when he learned of an unsolved killing in rural South Carolina.

When not living in Washington, Mr. Winthrop spent several months a year at a 25,000-acre family estate in Allendale County, S.C. Not far away, in the town of Fairfax, S.C., a young black man was shot and killed shortly after midnight on May 16, 1970.

When local law enforcement agencies failed to charge anyone in the case after two years, Mr. Winthrop launched what The Washington Post described as “a one-man crusade to find the killers and bring them to justice.”

Mr. Winthrop, who spent more than a year investigating the killing, died May 6 at his home in Washington. He was 73.

The cause was kidney cancer, his wife, Phoebe Jane Winthrop, said.

Beekman Winthrop launched a private investigation into a racially motivated killing in South Carolina in 1970, leading to the indictment of five men. (Family photo )

Warned by a state law enforcement official to stay away from the case, lest he be injured or killed, Mr. Winthrop was arrested for posing as a law enforcement officer. Charges were dropped. Members of his family also opposed his quest.

“My family jumped right through the wall when they found out I was involved,” Mr. Winthrop told The Post. “They’re conservative. They say it’s not our fight.”

But Mr. Winthrop decided to take up the fight for justice for 18-year-old Wallace Youmans, who was killed by a shotgun blast while walking past a white-owned store.

From a newspaper, Mr. Winthrop learned of the deathbed confession of a former town constable, who told local NAACP officials that he and five other white men had planned and carried out the killing. It was apparently in retaliation for the wounding of a white man, who was allegedly shot by African American assailants during a racial disturbance.

Mr. Winthrop spent more than $4,000 and interviewed 75 people before compiling a 110-page report, which he delivered in 1973 to the news media and to the Justice Department’s Civil Rights Division.

A month after receiving the report, the Justice Department reopened its investigation.

Within six months, five men were arrested in connection with Youmans’s death. Three were current or former police officers.

At their trial in 1974, two of the defendants were acquitted, and charges against the other three were dropped. But, according to news reports, Mr. Winthrop’s dogged pursuit of the evidence changed the racial climate in Allendale County and made him a hero to local black community. Some of the men put on trial quietly moved away.

Beekman Winthrop was born April 6, 1941, in Boston. His family’s fortune derived from banking.

Mr. Winthrop became interested in civil rights during his childhood in New York City, where he and a friend were the only white members of an otherwise all-black Boy Scout troop.

He was a graduate of the private Hill School in Pottstown, Pa., and received a bachelor’s degree in English from Columbia University in 1963 and a master’s degree in urban planning from Columbia in 1965.

After attending the Harvard Divinity School, Mr. Winthrop settled in Washington in 1969 and worked for the nonprofit Center for Community Change. He won a Robert F. Kennedy Journalism Award in 1972 for an article in the old New South magazine that showed how poor environmental conditions in South Carolina affected the health of children.

In later years, Mr. Winthrop worked for family businesses and as a private investment manager. He had homes in Washington and Luray, S.C.

Survivors include his wife of 44 years, the former Phoebe Jane Wood of Washington and Luray; a son, Dudley Winthrop of Washington; a brother; a sister; two half-brothers; two half-sisters; and two granddaughters.

One of the journalists Mr. Winthrop approached about the Youmans killing, Mark Ethridge of the Charlotte Observer, published a novel loosely based on the case in 2006. The book was the basis of a 2012 film, “Deadline,” starring Eric Roberts.

Mr. Winthrop explained that he took up the case of Wallace Youmans because “I was appalled that such a thing could still happen in the United States of America in the 1970s,” he told The Post in 1974.

“I guess we all feel we’d like to amount to something and do something meaningful with our lives. I felt that the Youmans case was my opportunity to step in and make a difference.”