MARIANA, BRAZIL (Reuters) -- Brazilian authorities early on Sunday confirmed a second death caused by a massive mudflow and flooding that swamped towns near an iron ore mine in the southeastern state of Minas Gerais.
As many as 28 people are still missing after the disaster on Thursday, prompting a rescue and salvage operation involving about 500 people, many of whom are still searching, with the help of dogs and special equipment, for victims along the floodplain downstream from the dams.
The torrent, carrying water and mud stained with mineral waste from the mine, flooded areas as far as 100 km (60 miles) away from the rupture. While the surge itself has receded, authorities are expecting the residue in the mainstream of the Rio Doce to reach the neighboring state of Espirito Santo by Tuesday.
Neither authorities nor mine operator Samarco - a joint venture between the world's largest mining company, BHP Billiton Ltd, and the biggest iron ore miner Vale SA -- have determined a cause for the rupture.
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Official response to the disaster has so far focused on recovery. But residents, regulators, environmentalists and others across Brazil have begun questioning oversight at the mine, pointing to broader concerns about the safety and sustainability of mining, one of Brazil's biggest industries and a major source of export revenue.
On Saturday, the Estado de S. Paulo newspaper reported that a study commissioned by the state government in Minas Gerais in 2013 had warned that the dams that burst could be vulnerable.
On Sunday, helicopters and vehicles went in and out of an operational base set up near the mine for recovery efforts that have been slowed by heavy rains and mist. Though finding survivors is growing less likely, officials said they did not rule it out.
Of the 28 people listed as missing, 13 were mine workers.
Duarte Junior, the mayor of Mariana, the city in which the mine is based, was admitted to hospital early on Sunday for what his wife said could be a heart attack, after nearly three days of emergency work.
The governor of Minas Gerais, Fernando Pimentel, who has called the rupture the worst environmental disaster ever to affect the mining-heavy state, said in a briefing on Sunday after flying over the region that the government would begin studying what regulatory measures may have fallen short.
Though dams in the state undergo reviews by independent inspectors, more would be needed to ensure similar disasters don't happen again.
"Obviously, this wasn't enough," he said. "We have to learn the lessons of this accident and improve the emergency plans."
Pimentel, however, sought to dispel notions that environmental licensing in the state could be to blame. "There was no failure on this front," he added.
The Samarco mine is located in the so-called iron quadrangle, one of the most heavily-mined regions in the world.
In the third quarter, according to output data reported by Vale, the Samarco mine produced 3.9 million tonnes of iron ore pellets, a 3.3 percent increase from a year earlier.
At a time when iron ore prices have collapsed compared with historic highs in recent years, cleanup and other costs related to the disaster, including regulatory penalties and any litigation Samarco may face, are expected to be high.