Updated at 5:20 p.m. ET

The death toll from an industrial accident has risen to 50 in Tianjin, a port city in northeast China. Emergency crews are still working to locate missing people; 12 firefighters are among the dead, and more remain unaccounted for.

Casualty numbers have been climbing, as rescuers find survivors and victims. Officials say more than 700 people have been injured and 71 of those are in critical condition. Firefighters and hazardous-material crews are still working at the site.

"The fire started at 10:50 p.m. local time and caused two powerful explosions that happened within 30 seconds of each other," as NPR's Laura Wagner reported for the Two-Way. The cause of that fire is still being investigated.

The blasts brightened the nighttime sky, sending mammoth fireballs into the air. They came from a warehouse holding chemicals and "dangerous goods," the state-run Xinhua news agency reports. Dramatic video from the scene shows streaks of flaming material arcing away from the main blast.

"The first one was equivalent to 3 tons of TNT," NPR's Frank Langfitt reports, citing Chinese media. "The second: 21 tons. Satellite photos suggest a blast radius of nearly 2 miles."

Hu Xiaoliang, a truck driver who was near the center of the blasts, tells Beijing News, "I was sleeping under my truck when the first blast occurred. It literally threw me out of the trunk. The second blast was bigger, flames came and now my truck is all ashes."

The powerful blasts toppled stacks of shipping containers, scorched hundreds of cars in nearby lots, and blasted out the windows of cars and buildings for a wide area. Tremors were felt more than 6 miles away. Smoke is still rising from fires in the area, and debris litters the roadways.

By Thursday evening local time, teams were still detecting high levels of sulfur dioxide, nitrogen oxide, and carbon monoxide from a third of a mile away, Xinhua reports.

Victims of the blast were using local schools for shelter Thursday. The People's Daily says that civilians and taxi drivers offered free rides to people who needed to get medical attention, and that many people in Tianjin, a city of more than 14 million, have lined up to donate blood.

"Industrial explosions like this are unusual in China, where workplace safety has markedly improved," Frank says. "The most recent disaster of this kind occurred in 2013, when more than 100 people were killed in an explosion at a poultry processing plant."

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