Six organizers and exhibitors involved with an outdoor Tokyo art festival in 2016 were referred to prosecutors Monday, over a fire at the event that killed a 5-year-old boy in a wooden jungle gym-like art installation.

The six, two of whom are university students, are suspected of professional negligence resulting in the death of the kindergartner, and injuries to two others, due to allegedly insufficient fire prevention measures. Police believe the fire was caused by an incandescent light bulb set inside the object.

The artwork, which was made by a student group at the Nippon Institute of Technology, contained wood chips that police concluded were ignited by the light, causing fire to spread throughout the structure.