At approximately 0606 hours on December 13, 1994, an explosion occurred in the ammonium nitrate plant at the Terra International, Inc., Port Neal Complex. Four persons were killed as a direct result of the explosion, and 18 were injured and required hospitalization. The explosion resulted in the release of approximately 5,700 tons of anhydrous ammonia to the air and secondary containment, approximately 25,000 gallons of nitric acid to the ground and lined chemical ditches and sumps, and liquid ammonium nitrate solution into secondary containment. Off site ammonia releases continued for approximately six days following the explosion. Chemicals released as a result of the explosion have resulted in contamination of the groundwater under the facility. The U. S. Environmental Protection Agency (EPA) Region VII was directed by EPA Headquarters to conduct an investigation to determine the cause of the explosion and to develop recommendations that would help prevent future similar occurrences in ammonium nitrate production facilities. This report contains conclusions reached by the EPA chemical accident investigation team regarding the cause of the explosion at the Terra International, Inc., Port Neal Complex that occurred on December 13, 1994, and recommendations for preventing future similar occurrences in ammonium nitrate facilities. This report is the culmination on ten months of work by EPA’s investigation team. The investigation was conducted principally by EPA On-Scene Coordinator (OSC) Mark Thomas PhD, Alan cummings, Dynamac Corporation; and Mariano Gomez, Ecology and Environment, Inc. Both Dynamac Corporation and Ecology and Environment, Inc , are EPA contractors. Dr. Thomas and Mr. Cummings participated in EPA’s emergency response activities at Terra that included assistance in air monitoring, chemical stabilization oversight. and tank integrity assessment.