According to a statement, the Tshwane Hazardous Material Unit was called in to inspect the envelope, resulting in the shutdown of electricity and the building being evacuated.
Vusi Mona, SANRAL's General Manager of Communication, said: "Our priority is to ensure the safety of the people who work at the COC."
He said the disruption would affect the Gauteng highway e-tolling system, and apologised to motorists for any inconvenience which this will cause.
"We take this matter very seriously and condemn this type of action in the strongest possible terms. It is tantamount to sabotage and borders on a very serious crime," said Mona.
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