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TIANJIN, China (CNN) — You can see the devastation everywhere: in the hollowed-out shells of buildings, in the anguished faces of relatives, in the parade of scorched cars.

But what set off the terrifying explosions that ripped through warehouses containing hazardous chemical materials, shooting fireballs across the sky and shaking buildings more than 2 miles away?

Hours later, amid the destruction in this northern Chinese port city of more than 13 million people, the cause of Wednesday night’s blast remained unclear.

A chemical odor hung in the air. Fires still burned in the waterfront industrial district where the explosions went off. And the grim toll kept mounting.

Among the 50 confirmed dead are 12 firefighters, officials said Thursday. More than 500 people are hospitalized, 71 in critical condition, the state-run Xinhua news outlet reported. Dozens of firefighters are missing.

Local authorities suspended firefighting efforts Thursday because of a lack of information about the “dangerous goods” stored at the warehouse at the heart of the blasts, Xinhua said.

The explosions originated at a warehouse site owned by Tianjin Dongjiang Port Rui Hai International Logistics Co., a company that stores and transports dangerous chemicals. Company executives have been taken into custody, state media said.

The blasts’ destructive force tore into Tianjin, smashing buildings and mangling shipping containers.

The first explosion was huge, and the second was even more powerful — the equivalent of 21 metric tons of TNT or a magnitude-2.9 earthquake, according to the China Earthquake Networks Center.

The explosions destroyed the house in which Qian Jiping and his wife, both of them migrant construction workers, were staying.

“When I heard the first explosion, I thought we were finished,” Qian said.

Strangers pulled them from the rubble. They fled barefoot, barely feeling the shards of glass that littered the ground.

Across the city, residents were jolted awake as the blasts shattered windows and fish tanks.

“The shock wave just blew through our apartment. It blew out the glass, it blew out the doors, it knocked out the power,” said Vafa Anderson, a teacher at an international school who lives less than 2 kilometers (about a mile) from the explosions’ epicenter.

Anderson told CNN he was awakened by the first blast and was looking out the window when the second went off, sending a “huge mushroom cloud” into the sky.

“I thought it was an earthquake,” said Liu Yue, a 25-year-old woman who lives about 4 kilometers (2½ miles) away. “I was extremely scared. I was afraid my family was in danger.”

She told CNN the 16-story building she lives in was rocking.

In a statement, the environmental group Greenpeace said it feared that the danger was not over.

“We are concerned that certain chemicals will continue to pose a risk to the residents of Tianjin,” the statement said.

“According to the Tianjin Tanggu Environmental Monitoring Station, hazardous chemicals stored by the company concerned include sodium cyanide (NaCN), toluene diisocyanate (TDI) and calcium carbide (CaC2), all of which pose direct threats to human health on contact. NaCN in particular is highly toxic. Ca(C2) and TDI react violently with water and reactive chemicals, with risk of explosion. This will present a challenge for firefighting and, with rain forecast for tomorrow, is a major hazard,” Greenpeace said.

Those injured in the blasts were taken to various hospitals in the city, with many reported to be suffering from cuts caused by broken glass.

People gathered outside one hospital not far from an area of badly damaged buildings, waiting for news of loved ones.

A severely burned man was wheeled past waiting crowds.

Some people collapsed from the heartbreak of losing someone close to them.

“Why did God take her? Why did God take my daughter?” one man cried out.

Will Ripley and Steven Jiang reported from Tianjin. Jethro Mullen reported and wrote from Hong Kong. Shen Lu, Dana Ford, Elizabeth Joseph and Don Melvin contributed to this report.