BEIJING — The young firefighters, their thin frames puffed up by insulated coveralls, some of them barely old enough to shave, rushed to the scene of a modest warehouse blaze in the port city of Tianjin on Wednesday night with little idea of the danger that awaited them.

Initial reports described a car on fire. But the 60 or so men who first arrived at the scene — contract firefighters employed by the Tianjin Port Group, many of them as young as 17 — confronted a blaze that had spread to metal shipping containers stored nearby. The firefighters aimed their hoses at the flames and turned on the water.

That turned out to be a deadly mistake.

Roughly 15 minutes later, an explosion — fueled by a collection of volatile chemicals that emit a combustible gas when wet — ripped through the warehouse. Yang Kekai, 27, was thrown to the ground as flaming debris rained down. Another blast, seconds later, sent him hurtling more than three yards. “When I was flying through the air, my heart skipped a beat and I thought I was finished,” Mr. Yang said later from a hospital bed.

Most of his colleagues were not so lucky.

At least 39 firefighters were among the 114 people killed by the cataclysmic explosions that injured more than 700 people and traumatized Tianjin, China’s third-largest city and the gateway to the country’s industrial northeast. At least 70 people remained unaccounted for, most of them private firefighters employed by the company that runs Tianjin’s sprawling port.