Rob Johnston, who is overseeing the federal Transportation Safety Board's investigation into the derailment, said seven of the cars travelling with that train were Class 111.
Six of them were carrying hazardous products — hydrochloric acid, sodium hydroxide and petroleum distillates (a varsol-type substance).
"The two that were carrying petroleum distillates did end up eventually releasing their products, and that was probably the source of the fire," Johnston said.
No one was hurt in the Saskatchewan derailment, in which 26 out of 100 cars went off the tracks, but officials noted the outcome could have been a different if it had happened in a populated area.
Following the July 2013 Lac-Mégantic derailment, which resulted in 47 deaths and destroyed more than 30 buildings in the Quebec town, the TSB issued a report that found the Class 111 tank cars were outdated.
It said "enhanced protection standards must be put in place" for Class 111 tank cars. Damage to the Lac-Mégantic tank cars could have been reduced by enhanced safety features, the report said.
The TSB told CBC News it does not know whether the cars in Clair were given safety upgrades or not.