The explosion in the production plant of the Pliva pharmaceutical company in Zagreb on Friday was caused by a static electricity spark, Zagreb Police said on Sunday.

The spark set on fire the mixture for the production of the Plivadon medicine on the 5th floor of the Pharmaceutical Technical Operations building.

The accident took place around 3pm Friday. The fire was contained and put out by the company's fire-fighting unit.

Nine people were injured and have been hospitalised. Two people sustained severe burns in the explosion, the Zagreb Trauma Centre reported, adding that the condition of one person was still critical.

After examining the explosion site, an environment protection inspector said the explosion caused no threat to the environment and the population.

Pliva communications director Tamara Susanj Sulentic told the press that foreign experts would also be included in the investigation of the explosion, adding that the production would be shut down until the investigation was completed.