JOHANNESBURG — A high-ranking politician in South Africa’s governing African National Congress was arrested in connection with the assassination of a party member who had exposed corruption in a public works project, officials said Monday.

Mluleki Ndobe, one of the most influential A.N.C. power brokers in KwaZulu-Natal Province, was arrested early Sunday for suspected involvement in the mafia-style killing of a nationally known party leader, Sindiso Magaqa, according to party and police officials.

Mr. Magaqa, who had loudly condemned the theft of public money in the town of Umzimkhulu, was fatally shot inside his Mercedes-Benz in 2017 by two hit men who sprayed the car with gunfire. A man suspected of being one of the killers was arrested in September.

The assassination — the most high-profile in a wave of political killings that have swept across South Africa since early 2016 — came to symbolize the decline of the A.N.C., whose members have increasingly targeted anticorruption whistle-blowers and turned on one another in lethal fights over political posts.