City authorities have shortlisted three potential reasons for the earlier blast in downtown Johannesburg, the media reported, citing the city manager Floyd Brink.
According to the official, the first possibility is that methane gas may have been ignited by sewage entering the underground rainwater system.
The second option proposes that the natural gas getting mixed up with oxygen in underground service ducts may have led to the emergency situation.
As for the third assumption, the destructive blast could have been caused by a ruptured gas pipe.
"At this stage only one of the above potential causes appear to be warranted, accidental leakages of natural (gas) into the service duct reaching explosion concentration levels of between 5% to 15%, and was ignited by a source unknown at this stage. The source of gas in the service duct is unknown. Investigations in this regard will continue to try locate the source over the next few days," Brink said.
On Wednesday, an explosion in Johannesburg severely damaged the city's infrastructure and injured many people. According to local media, a large pit was left in the main road after the explosion.
As for the casualties, at least 41 citizens were injured. The majority (30 people) were slightly harmed; nine victims suffered serious injuries and two of them are in critical condition, the media reported, citing Panyaza Lesufi, the premier of the Gauteng province's government.