The dam's rupture on November 5, 2015 near the town of Mariana unleashed a giant torrent of toxic mud that swamped villages, rivers and rainforest, killing 19 people on its way to the sea.

Scientists say the sludge caused "permanent" pollution on the river Doce and its coastal plain.

Brazil's government filed a criminal complaint against the mining companies and several of their executives over the spill.

But a court in Belo Horizonte, capital of southeastern Minas Gerais state, where the disaster occurred, ruled that state prosecutors had failed to prove that "individual behavior contributed directly and decisively to the collapse of the dam.

"And, in the context of the criminal trial, the doubt... can only be resolved in favor of the accused," the judge wrote.

Pamela Rayane Fernandes, mother of Emanuele Vitoria, a five-year-old girl who died in the tragedy, reacted with disappointment to the companies' acquittal.

"Although I expected this response from the Brazilian justice system, it was still a shock to know that in the place where we live, where we come from, they (the state) cannot give us protection," she told AFP.

- $30 bn in damages -

The ruling comes nearly three weeks after Australia's BHP and Brazil's Vale reached a deal with Brazil's government to pay nearly $30 billion in damages over the collapse of the tailings dam at mine operated by Samarco.

The payout is the biggest of its kind for an environmental disaster, according to Brazilian President Luiz Inacio Lula da Silva.

The companies agreed to pay 100 billion reais (17.5 billion dollars) to local authorities over twenty years and 32 billion reais ($5.6 billion) towards compensating and resettling the victims, as well as repairing the harm caused to the environment.

Thursday's court decision also comes a month into a mega-trial in London over BHP's role in the mudslide.

More than 620,000 complainants, including 46 Brazilian municipalities and several Indigenous communities, are seeking an estimated pound36 billion ($47 million) in damages from the company, which denies liability.

The dam's failure released a torrent of over 40 million cubic meters of sludge, the equivalent of 12,000 Olympic swimming pools, which flowed through the Doce river channel all the way to the Atlantic Ocean, over 600 kilometers (373 miles) away.

It killed thousands of animals and left over 600 people homeless.

Scientists say the mouth of the Doce River and parts of the southeast Atlantic coastline are still contaminated with metals from the spill, affecting the area's population of fish, birds, turtles, porpoises and whales.

BHP and Vale had already agreed in 2016 to pay 20 billion reais (about $3.5 billion at today's rate) in damages, but the negotiations were reopened in 2021 due to what the government called their "non-compliance".