The collapse of a massive tailings dam in Brazil was caused by the accumulation of water and lack of drainage, although the facilities did not previously show signs of instability, according to a report commissioned by the dam's owner and published Thursday.

The January 25 dam breach in the mineral-rich state of Minas Gerais spewed millions of tons of toxic mining waste across the countryside, leaving 270 people dead or missing and forcing owner Vale to suspend some of its operations.

A group of specialists hired by the Brazilian mining giant found "the rupture and resulting sludge slide was due to the static liquefaction of the waste from the dam," meaning the solid dam waste became sludge due to the accumulation of water.

According to the specialists' report, the dam had insufficient drainage and accumulated water in the middle of the rainy season. As a result, it was filled with "soft" and "heavy" waste, due to its high iron content.

The growing pressure created a "marginally stable dike" -- in other words, it was "near the breaking point" because it was almost unable to drain water.

The report added that the dike "showed no signs of instability, such as large deformations that caused cracks or swelling before the break."

Although subsequent data analysts identified small deformations over the 12 months leading up to the break, the deformations were "too slow to be detected by ground radar and other surveillance devices," the report said.

Emergency crews use an amphibious vehicle and heavy machinery in May 2019 to pick through mud and search for bodies, 99 days after the collapse of the Corrego do Feijao Dam in Minas Gerais
Emergency crews use an amphibious vehicle and heavy machinery in May 2019 to pick through mud and search for bodies, 99 days after the collapse of the Corrego do Feijao Dam in Minas Gerais AFP / Douglas MAGNO

The report made no reference to the results of other investigations.

A November 5 report by the National Mineral Agency (ANM) show the mining company was aware of problems with a horizontal drainage system installed seven months before the disaster.

On January 10 -- two weeks before the dam ruptured -- two measuring devices showed liquid pressure had reached emergency levels.

At least seven Vale workers, none of whom are senior managers, and six employees of the German auditor TUV SUD are under investigation for using false information to certify the Brumadinho dam met safety requirements.

ANM's damning accusations came on the fourth anniversary of the collapse of another tailings dam owned by a joint venture between Vale and the Anglo-Australian miner BHP.

The failure of the Fundao tailings dam unleashed a torrent of nearly 40 million cubic meters of highly toxic mine sludge over areas of the Mariana district, also in Minas Gerais.

The mud killed 19 people and flooded 39 towns, marking Brazil's worst environmental disaster.