Two men walked into the National Union of Mineworkers office at the Lonmin platinum mine in Marikana, South Africa, and shot two officials on Monday of this week.
A shop steward was killed and the branch treasurer is critically ill in hospital.
Last month a leading activist in the more radical AMCU union was shot dead in a nearby bar.
Police massacred 34 striking miners at the mine last August.
Attempt to oust ANC critic falters
Zwelinzima Vavi, general secretary of South Africa’s union federation Cosatu, survived an attempt to oust him last week.
A no confidence vote was abandoned at the last minute.
This is part of a row that combines anger at corruption with bitterness over the unions being too closely identified with the ANC government.
Vavi is accused of corruption, but supporters say he is under attack because he is critical of the ANC.
The ANC seems to have told its supporters to back off, worried that the row will hurt its chances in next year’s election