The first extensive chemical attack by Iraqi troops was carried out in March 1984, when they used tons of sulfur mustard and nerve agents against Iranian soldiers on the Majnoon Islands battlefields (along the northern border).  Afterward, extensive employment of chemical weapons by Iraqi troops in March 1985 led to huge Iranian casualties both soldiers and volunteer combatants.
Following many requests by the Iranian Government, three official investigation team were sent to Iran in March 1984, February/March 1986 and April 1987.  The conclusions, based on field inspections, clinical examinations of casualties, and laboratory analysis of samples, were released as three official UN reports (S/16433, S/17911, and S/18852).  Based on the UN fact finding team's investigations they confirmed the use of mustard gas as well as nerve agents against Iranian troops.