NHAN TRACH, Vietnam >> Since a devastating fish kill blighted the waters along 120 miles of coastline in central Vietnam, hundreds of people are believed to have fallen ill from eating poisoned fish. Here in the fishing village of Nhan Trach, the squid that sustain the local economy have virtually disappeared. And a fishing ban has left hundreds of traps sitting unused on the beach and dozens of small fishing boats idle.

“We are so angry,” said Pham Thi Phi, 65, who operates a fishing boat in Nhan Trach with her husband and three grown sons. “If we knew who put the poison in the ocean, we would like to kill them. We really need to have an answer from the government on whether the ocean is totally clean and the fish are safe to eat.”

While the immediate cause appears to have been toxic waste from a nearby steel mill, fury over the episode has exploded into a national issue, posing the biggest challenge to the authoritarian government since a spate of anti-Chinese riots in 2014. Protesters demanding government action have marched in major cities and coastal communities over the past six weeks, escalating what had been a regional environmental dispute into a test of government accountability and transparency.

But two months after the fish started washing up on beaches here, the government has yet to announce the cause of the disaster or identify the toxin that killed marine life and poisoned coastal residents.

The government’s failure to respond and its previous support for the Taiwan-owned steel plant at the heart of the crisis have fueled widespread suspicion of corruption, cover-up and the hidden influence of foreign interests at the expense of Vietnamese livelihoods, a potent mix that challenges the legitimacy of Communist Party rule.

“Quite simply, in Vietnam, human life is less important than the political life of the government and government institutions,” said Nguyen Thi Bich Nga, an activist in Ho Chi Minh City. “In this way, we can explain all that is unusual in this country.”

The government has said little about the marine die-off while cracking down on the protests, which have been called every Sunday since May 1, when thousands of people took to the streets of Hanoi, Ho Chi Minh City and five additional cities. More than 500 people have been arrested, and demonstrators have been beaten by the police.

“The response by the government has been one of ineptitude,” said Carlyle Thayer, a Vietnam analyst at the Australian Defense Force Academy. He said the fish kill was the most serious environmental issue to confront the government in several years and reflected poorly on the government of Prime Minister Nguyen Xuan Phuc, who took office in April.

Last month, the Office of the United Nations High Commissioner for Human Rights urged the government to avoid excessive use of force, citing “increasing levels of violence” against the protesters.

But the protests have continued.

On Sunday, more than 1,000 people turned out in a coastal district of Nghe An province, north of the steel plant, to demonstrate. Many wore T-shirts bearing a fish skeleton. Some carried signs reading, “Fish need clean water, citizens need transparency.”

“It seems the government tries to cover up for the culprit,” the Rev. Anthony Nam, a Catholic priest and protest leader in Nghe An, said by telephone. “We will protest until the government says what caused the spill.”

In Nhan Trach, about 40 miles south of the steel factory, the dead and dying fish first appeared in early April, floating in the surf and washing up on the beach. Initially, it seemed like a windfall, and many people here ate and sold them. The fish kept coming, tons of them, day after day for more than a month, residents said.

“Some of the fish were dead; some were dying,” said Ho Huu Sia, 67, who buys and dries fish for a living. “We ate the fish that were still alive. We ate the fish for two weeks.”

His daughter, Ho Thi Dao, 32, said she became ill, experiencing vomiting, diarrhea, headaches and dizziness. She went to the local clinic and received intravenous fluids. She said she met others there who also suffered poisoning.

“I ate the fish and got poisoned,” she said. “Many people got sick like me.”

Belatedly, the government announced that aquatic life had been poisoned along the coastline of four provinces. Authorities warned people not to eat fish and ordered a halt to fishing.

As compensation, officials distributed bags of rice and gave fishermen 50,000 dong, or about $2.20.

“We are just sitting with tears running down our cheeks looking out at the ocean,” said Phi, who has been fishing from Nhan Trach all her life. “What can we do with 50,000 dong?”

Coastal residents and journalists quickly identified the culprit as the Formosa Ha Tinh Steel plant, which opened in December.

According to news reports, the fish kill happened after the factory washed unspecified cleaning chemicals through its wastewater pipeline. A company representative seemed to confirm the suspicions in April when he said it would not be surprising if the factory’s wastewater harmed marine life.

“You have to decide whether to catch fish and shrimp or to build a modern steel industry,” he told reporters. “Even if you are the prime minister, you cannot choose both.”

His comments incited a flurry of criticism on social media and spawned a popular hashtag, #ichoosefish, which has become a protest slogan.

The company later insisted that it met Vietnam’s environmental standards and said that the spokesman had been fired. Company officials did not respond to requests for comment.

The government has been just as reticent. At first, the government suggested a toxic algae bloom was responsible. In mid-May, Pham Cong Tac, deputy science and technology minister, told Vietnamese news outlets that the ministry had a “convincing scientific basis” to explain the fish deaths, but he did not disclose what it was.

Last week, Mai Tien Dung, minister and head of the government office, said that the authorities had identified the cause but indicated that officials could not tell the public because an investigation was continuing.

“The work of identifying the cause of the dead fish is also related to identifying the culprit,” he told reporters. “This not only needs scientific evidence but also complete evidence of a legal violation, especially of environmental law.”

The lack of information has only fueled the protesters’ anger.

Villagers say authorities collected water samples immediately after the episode, and foreign experts say test results should have been known within days.

Nguyen Hoang Anh, a university professor in Hanoi, said the government should have immediately revealed the toxin, especially to the poisoning victims and their doctors.

“It’s not fair,” she said. “It’s not ethical. It’s a crime.”

She said the cover-up had the potential to make the fish kill Vietnam’s Chernobyl, the 1986 nuclear disaster that contributed to the unraveling of the Soviet Union.

That is what the government most fears, analysts say, and it is why it acts quickly and at times brutally to suppress protests before they ignite a popular uprising.

But critics say the government has another motive for wanting to bury the controversy.

The government has supported the steel plant, giving the company a sweetheart deal, including tax incentives and a bargain price for the property, to build on the coast.

The company, a subsidiary of the Formosa Plastics Group, paid $4.3 million for a 70-year lease on 8,150 oceanfront acres, according to Vietnamese news reports. That is about $530 an acre.

To clear the site for construction, the government relocated nine communities with more than 14,000 people. In 2012, the prime minister at the time, Nguyen Tan Dung, attended the groundbreaking ceremony for the project, which includes a seaport and a power plant.

“Some important people in the government made a corrupt deal to put the factory there, and it is therefore partly responsible for the spill,” Huynh Ngoc Chenh, former editor of the Thanh Nien newspaper and a prominent blogger, said in an online interview. “So it can’t easily blame Formosa or take responsibility. So it is saying nothing and cracking down on protests.”

Two years ago, while the factory was under construction, it became a prime target of the riots over China’s placement of an oil rig in waters off Vietnam in the South China Sea. More than 200 factories owned by Chinese and other foreign companies were looted and set ablaze around the country.

But the worst rioting occurred at Formosa, where four people were killed. The company is based in Taiwan, but thousands of imported laborers from mainland China were building the factory. Protesters stopped buses, pulled off Chinese passengers and beat them.

Authorities have been more careful not to let the current protests get out of hand. But even if they can be quelled, the economic costs have continued to mount.

On a recent morning, more than a dozen fish traders gathered at a drink shop across from the beach here. A few played board games, and others joined in a spirited game of cards. There was nothing to do but kill time, one said.

Around the corner, Phan Dinh Son, 49, sat at a table in his all-too-quiet open-air shop. He used to sell hundreds of blocks of ice a day. Now he sells about 20, he said. A separate business buying and trading shellfish has been suspended because no one wants to eat local fish.

“The fish market is empty,” he said. “I would hope the government and the party would come up with a solution and give a clear answer so we can do our business.”

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