It was the worst accident in Houston history. On May 11, 1976, at 11:08am, a tanker carrying 7,500 gallons of anhydrous ammonia lost control over the busy Loop 610 at Highway 59 (now I-69) interchange. The truck crashed through a guardrail and careened off a ramp to the freeway below. The tractor-trailer struck a column on the way down, rupturing the tank and releasing a deadly cloud of ammonia over the Southwest Freeway. Those who weren't injured in the initial accident found themselves trapped in a cloud of gas and running for their lives. One person was killed in the accident and five people died from inhaling anhydrous ammonia. There were 178 people injured from the ammonia fumes, including 78 who had to be hospitalized. The National Transportation Safety Board concluded that the probable cause of the accident was the excessive speed of the vehicle combined with the lateral surge of liquid in the partially loaded truck, which caused it to overturn.