JOHANNESBURG — A top corruption inspector in South Africa has announced that she will investigate two high-ranking African National Congress politicians in a case related to the abuse of public funds for a dairy farm — a rare sign that powerful members of the governing party could be held to account for endemic corruption under former President Jacob Zuma.

The inspector, Public Protector Busisiwe Mkhwebane, said on Tuesday that the inquiry would focus on Ace Magashule, secretary general of the A.N.C., and Mosebenzi Joseph Zwane, the former minister of mineral resources — the two officials behind the dairy farm project in Vrede, in the province of Free State. Prosecutors describe the project, which was meant to help struggling black farmers, as a fraud that funneled $21 million to the A.N.C.’s business allies.

Since Mr. Zuma was ousted from power in February and replaced by Cyril Ramaphosa, the authorities have turned their attention to the dairy farm case, which has come to symbolize widespread corruption under the A.N.C. and its betrayal of poor black South Africans.

The police have apprehended some low- and midlevel government officials, as well as foreign businessmen, involved in the dairy farm, the first arrests related to a high-profile case of public corruption during the Zuma years.