South African construction firm Murray & Roberts Holdings has started a probe into the collapse of a bridge support structure over one of South Africa's busiest motorway, the company said on Thursday.
Emergency services officials said two people were killed during the incident while 21 were injured.
"The cause of the incident has not yet been determined and the company is working closely with the relevant authorities on the investigation," Murray & Roberts said in a statement.
Independent investigators have been appointed, but finding the cause of the collapse could take weeks, if not months, said Eduard Jardim, a spokesman for the company.
Shares in Murray & Roberts recovered somewhat after a 7 percent drop on Wednesday, climbing 4.8 percent to 11.69 rand by 0817 GMT, compared to a 1.2 percent rise in the Johannesburg Securities Exchange's Construction and Materials index.
The mangled structure sprawled across Johannesburg's M1 highway near a busy offramp leading to the Sandton financial district, caused hours of delays for motorists during evening peak traffic on Wednesday and again on Thursday morning.