Investigators say picric acid is blamed for sparking an explosion at Rozzi's fireworks Friday.   

It happened while a building was being demolished on Lebanon Road in Symmes Township.

The picric acid apparently spilled onto the concrete of the building, then crystallized.

The unstable chemical ignited some old flares in the building. 

Two people were taken to Bethesda North Hospital for burns.  Both are now back to work. 

No charges are expected to be filed in the incident.

Six nearby homes were evacuated as a precaution.