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.