News of three fatal explosions involving refrigerated shipping containers briefly shut down the Horizon Lines cargo dock in Unalaska last year. Now, Russian authorities say they’ve seized a shipment of contaminated coolant they believe is responsible for the exploding “reefers.”

Refrigeration trade publication ACR says customs officers in St. Petersburg found more than a thousand cylinders of coolant marked as Freon that in fact contained the highly flammable and explosive gas methyl chloride.

The shipment originated in China and passed through Poland on its way to Russia. ACR reports that an investigation is ongoing.