The ANC has strongly condemmed what they are calling an act of arson after a fire broke out at the party's headquarters on Tuesday.
"The bottle was strategically placed under the couch at reception – a bottle filled with chemicals. The couch caught fire as the bottle exploded," said Mthembu on Wednesday.
"In our books that is grounds to be confident that this was arson," he said.
He said the chemicals that filled the bottle left fumes that were still lingering in the building. Mthembu added that a bomb unit, police and other experts all confirmed that the attack was indeed an arson attempt.
"It is pretty clear what we are dealing with and we view such actions in a serious light."
The ruling party expressed disappointment at the act and said such actions did not have a place in a democratic nation.
"We have avenues to address issues and this [attempted arson] is not one of them…This was a criminal act.
"South Africa does not need such incidents. An attack at the offices of the governing party or any political office under a democracy is something we do not need."
SA Police Service Brigadier Neville Malila confirmed an investigation into an alleged arson attack was underway.
Mthembu applauded security staff on duty.
"Security personnel used fire extinguishers to put down ballooning flames that were almost reaching the ceiling of the reception area," he said.
The ANC said it was unclear what motives were behind the attack.
Work at Luthuli House would continue as usual on Wednesday.– Sapa