At least 65 people remain unaccounted for and presumed dead from the disaster, most of them firefighters.
Throngs of curious onlookers gathered on Thursday on the banks of the Haihe River to take pictures of the grim scene of dead fish, while officials tried to reassure the public. Mass die-offs are not unusual in the summer, when oxygen levels in the polluted river water can fall sharply, the officials said.
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The state-owned China Central Television reported later that tests found no significant presence of cyanide in that part of the river.
The official assurances seemed to fall on deaf ears.
"I've never seen anything like it," said Wang Lei, 47, a freight company manager who wore a face mask as he surveyed the mass of dead fish clogging the river's shallows. "There has to be a link between the dead fish and the blast. What else could explain the death of so many?"
Photos of the dead fish were posted widely on Chinese social media, feeding public worries over the tons of chemicals that were in the warehouse. The authorities have already said the logistics company that operated the facility broke the law by storing such dangerous materials too close to apartment houses, highways and public buildings.
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Chinese officials are contending with a public that is losing patience. Residents of the area near the blast site have staged protests, demanding compensation for their damaged homes, while the anguished families of firefighters who first responded to the blaze sought word on their missing loved ones.
At a Politburo meeting on Thursday, President Xi Jinping promised a full investigation of the disaster.
"The incident has caused heavy casualties and property loss," he said, according to a statement released after the meeting. "It was a profound lesson paid with blood."
Officials have said that the company, Rui Hai International Logistics, was storing at least 2,500 tons of hazardous chemicals in the warehouse.