Taiwanese prosecutors have dropped all charges against a group of executives at the waterpark where an explosion of flammable coloured powder killed 15 people and severely burned hundreds of others.

More than 500 people were injured and 15 people died when the starch-based powder ignited during the Color Play Asia event at the Formosa waterpark in New Taipei City on 27 June 2015. The flames engulfed a large portion of the dance area almost instantaneously.

Charges against Formosa waterpark chair Chen Po-ting – one of eight people connected with the blast – were dropped after prosecutors said he could not be held responsible as he was only in charge of general operations and not in the planning of the event.

In addition to Chen Po-ting, Formosa executives Chen Hui-ying and Lin Yu-fen had charges dropped against them, as did the manager and four employees of the company that leased the space to hold the colour party. Among those dismissed included two workers responsible for blowing the colour powder from the stage into the crowd.

To date, Lu Chung-chi – the man responsible for organising the waterpark party – is the only person to have been prosecuted, receiving a jail sentence of four years and 10 months for his role in the tragedy.

Fire officials are still investigating the cause of the explosion, which is suspected to have been caused by a mixture of air and concentrated combustible particles.

Then-Premier Mao Chi-kuo ordered a ban on colour powder events for the duration of the investigation. Since the incident, no colour powder events have been held in the country.