The ANC was unaware that it was harbouring "a criminal" when it employed a man who was under investigation for alleged links to a cash-in-transit heist, the party's elections head Fikile Mbalula said.
The party now wants all of its staff to be vetted.
Speaking to the media on the sidelines of the second day of the ANC KwaZulu-Natal elective conference in Durban on Friday, Mbalula said Errol Velile Present had denied any criminal involvement.
"In the ANC, we train a lot of young people. Some of them come in there and we train them and give them responsibilities. The member that has been arrested as a suspect in the case of a cash-in-transit [heist] in Johannesburg is also in that pool.
"We wouldn't know at any given point in time when we give people volunteer services in our party headquarters that we are harbouring criminals, and hardened criminals for that matter," said Mbalula.
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Mbalula said Present himself, who was a staffer at the party's headquarters at Luthuli House in Johannesburg, "said that he was not involved in criminal activities".
"But look now, we are harbouring a criminal," Mbalula added.
Present appeared alongside three co-accused in the Roodepoort Magistrate's Court on Tuesday.
'Veli was not a permanent worker'
Mbalula now wants everyone who works at the party's headquarters, including temporary staff and volunteers, to be vetted.
"We only vet permanent employees, we don't vet temporary people who come there to get skills and then from there go elsewhere to find employment for themselves. But now it means everybody who works for us at our headquarters, we must know their backgrounds, what they do and what their intentions are," he said.
He added that the party needed to be vigilant of people who wanted to use it as a "hideout" from their criminal activities.
Mbalula also stressed that Present was not a permanent employee "as it has been portrayed".
"We should be very clear, Veli was not a permanent worker of the ANC. He came there, and he would volunteer to go into the organising department to do work in relation to mobilisation and from time to time, he would get a stipend," said Mbalula.
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He suggested that Present tried to use his involvement at Luthuli House "as part of his cover" during his arrest.
"Police must do their jobs. [If] a criminal is a wolf in a sheep's skin, they must be caught and be brought to book."
He added that police officers had done a "marvellous job" by catching the suspects.
"The courts would now decide, but as the ANC, we are very clear that we can't harbour criminals or suspects in our ranks who are involved in such criminal activities. That's why we are very clear that we have shown him the door," he said.