"We have questioned seven witnesses," Chief of the Jakarta Metropolitan Police Inspector General Gatot Eddy Pramono said.
No suspect has been named in the case as the police were still questioning the witnesses, he said.
The police will send a forensic team to the scene to conduct further investigation after the fire was put out, he said.
According to the preliminary result of investigation, the fire was caused by welding sparks.
No injuries and fatalities were reported.
Reporting by Fianda Sjofjan Rassat, Suharto