The fire that killed one young boy and injured two adults at Tokyo Design Week on Sunday was likely started by an incandescent bulb.
The Japan Times reports that the light bulb was placed inside the installation that caught alight at an exhibition of student work.
Officials from the Nippon Institute of Technology, whose students made the installation, admitted that the light had been placed on the ground inside the structure among woodchips.
The university had initially claimed that only safer LEDs were used at that site, but it has since revised that statement. Officials now believe that a student left on an incandescent bulb that had been intended for use during construction only and not during public exhibition.
Nippon Institute of Technology president Kenichi Narita told a news conference on Monday that "there was a gap in recognition among students regarding the risk" of using the lamp.
"The responsibility for the accident solely lies with the university and its president," said the university, which is awaiting the results of an investigation by the police and fire department before it confirms a link between the light and the fire.
Police are reportedly investigating the accident on suspicion of professional negligence resulting in death and injury.
The installation – a jungle-gym-like climbing tower – was made of timber and paper.
Five-year-old Kento Saeki had been playing on the structure and died when he was trapped inside by the fire. The other injured parties were his 44-year-old father and another man in his 40s, who both suffered burns trying to save the boy.
Tokyo Design Week events were cancelled on Monday, which was due to be the last day. The trade show is held each year at the Meiji Jingu Gaien gardens in the Japanese capital.
In a statement released on Monday, Tokyo Design Week representatives expressed their apologies and intention to co-operate with police and fire department investigations.
"We deeply apologise to those who passed away and to all the bereaved families from the bottom of our hearts," they said.