And as the probe continues, it is unclear how much damage was caused to the five-storey Braampark building, which was gutted by flame and may have suffered structural damage.
Johannesburg Emergency Management Services spokesperson, Synock Matobako, said fire and rescue personnel were now on the way to the scene.
"We searched for any other people who may have been trapped inside the building".
Snaps of the blaze, which has engulfed the Forum 1/A building, began appearing on the social network around 2pm SAST. Matobako confirmed that due to swift evacuation, no one was injured. "On arrival we found that the fire was well alight and the roof totally collapsed", Matobako said.
"The fire was still ablaze at the time investigators arrived".
There were about a thousand workers in the building when the fire broke out.
It is alleged construction had been taking place on the building's roof. "The ground floor wasn't damaged because they could extinguish the fire in time".
He asked not be named as he is not authorised to speak to the media.
Police say EMS' inspectors and firefighters will hand over the scene to the SAPS once they've wrapped up their own sweeps.
Firefighters at the building after the fire.
Matabakho said it also didn't make sense that such a large building had just one fire hydrant.
"I saw contractors. who had been working on the roof, running away".
An employee who works on the top floor said they were fortunate to have escaped. "There was dust before then", she said.
"We were forced to use fire hydrants in the street because we had problems with the building's fire hydrants‚" he told TimesLive.