Police spokesperson Brig Motlafela Mojapelo said that Kekana was shot at point blank range by a gunman who subsequently fled the scene, evading arrest.
Twala called for a robust response to the violence from South Africans, saying that "political hyenas must be stopped before it is too late".
"These political hyenas think that they can strike fear into the ... forces of our national democratic revolution by hiring assassins to silence whistle blowers within our movement and society," he said.
Twala said that the murders would only motivate the union to galvanise its efforts to root out corruption.
"Our campaign against corruption and looting will never be halted by guns," he said. "If anything, these assassinations are going to heighten our resolve to embark on militant campaigns to hound out corrupt elements within our movement and government."
He called on the public to help in the fight against corruption in the interest of preserving "our hard won democracy, freedom and good governance".
"Beneficiaries of state capture, corruption and VBS looters will never intimidate the collective power and determination of our people in fighting corruption," he said.
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