Fire investigators in Tacoma, WA, say they think a massive explosion at a foundry was caused by a faulty hose, or faulty connection between a propane truck and a stationary tank.

According to media reports, firemen think the bad connection let propane vapors escape and drift to nearby furnace on Monday, Oct. 8.

Four people were injured in the blast – one critically – and 13,000 area customers lost electricity when the explosion knocked out a substation.