RIO DE JANEIRO, Jan. 29 (Xinhua) -- Brazilian authorities on Tuesday arrested five people in connection with the iron-ore waste dam collapse last week, which killed scores and causing an environmental catastrophe.

Police arrested three employees of mining giant Vale, which owns the mine in Brumadinho in the southeast state of Minas Gerais, and two subcontracted engineers.

The employees "were directly involved in the licensing of the dam that collapsed," and the engineers "had declared the dam was stable," state news agency Agencia Brasil said.

While arresting the suspects, who will be detained for a period of 30 days, police also gathered documents as evidence.

"It is hard to believe that a dam with this size -- run by one of the world's biggest mining firms -- can burst unexpectedly, without any sign of vulnerability," said Perla Saliba Brito, the federal judge that ordered the arrests.

The dam was reportedly equipped with sensors capable of detecting signs of structural weakening.

The incident marks the second tailings dam collapse in three years at a Vale-owned mining operation in Brazil, flooding communities and fields with toxic sludge.

In November 2015, a tailings dam partly-owned by Vale ruptured in Mariana, also in Minas Gerais, destroying an entire community and killing 19 people. It was considered Brazil's worst environmental disaster.

Founded in Brazil in 1942 and privatized in 1998, Vale became the world's largest iron ore producer in 2013.

In total, Vale accounts for 70 to 80 percent of Brazil's iron exports, which reached 20.2 billion U.S. dollars in 2018, and ranks as the world's second-largest exporter of the mineral.

The Brumadinho mine produces 8.5 million tons of iron ore a year, or 2 percent of the company's total annual output.

Industry observers said the company's success is partly due to the fact it pursued contracts "aggressively," but Brazil's competitive prices and the quality of the mineral were also factors.

Officials have put a freeze on Vale accounts to the tune of 11 billion reals (2.9 billion U.S. dollars) to guarantee funds for indemnification.

But critics are skeptical, saying Vale has yet to be held accountable for the 2015 dam disaster.