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RIO DE JANEIRO—Nearly 10 months after one of the biggest disasters in the history of mining, the companies responsible for the failure of a massive tailings dam in Brazil presented a long-awaited report Monday on the causes of the incident.

But it added little information to what Brazilian police and prosecutors already had gathered, and it presented no guidance on how to prevent future accidents.

The Fundã o dam failure at mining giants Vale SA's and BHP Billiton Ltd.'s Samarco joint-venture in Brazil is believed to have been the largest such accident.

Its Nov. 5Â collapse released enough mine waste to fill the Dallas Cowboys' AT&T Stadium at least 11 times. The waste destroyed villages and killed 19 people as it rushed down one of southeast Brazil's main river systems, polluting more than 400 miles of waterways along the way.

"My commitment is to learn every lesson we can from what happened here," BHP Billiton Chief Executive Andrew Mackenzie said Nov. 16. "We will be entirely open with the findings of this report to hope that regulators and companies around the world benefit from that learning, so that something like this never happens again."

The 88-page report released Monday, however, explained little more than the mechanics of the collapse, which Brazilian police outlined in June.

Fundã o, an earthen dam, had been experiencing drainage problems since at least 2009. A change to the dam's design in 2012 worsened its drainage and left a key section of the dam's face resting on top of unstable, silt-like mine waste known as "slimes," said the report's lead author, renowned Canadian mining engineer Norbert Morgenstern.

A series of very small earthquakes on Nov. 5 likely caused the entire structure to turn to mush, a geotechnical phenomenon called "liquefaction flowslide," Dr. Morgenstern said in a presentation on the report's findings.

Left unsaid were what Brazilian authorities and many independent experts have described as serious flaws in Samarco's risk management and decision-making that led the company to continue mining—and depositing waste into Fundã o—despite clear signs of structural instability in the years before the disaster.

During an hour-long press conference, Dr. Morgenstern declined to answer questions about what could have been done during Fundã o's design, construction and operation to prevent it from collapsing.

Reporters weren't allowed to direct questions to Vale CEO Murilo Ferreira, Samarco CEO Roberto Carvalho or BHP Billiton Chief Commercial Officer Dean Dalla Valle, who made brief opening and closing statements but otherwise sat silently in the room while Dr. Morgenstern presented the report.

Industry critics seized upon the report as evidence that mining companies are trying to save their own necks rather than prevent future disasters.

Lindsay Newland Bowker, an environmental risk manager in Maine who studies accidents at mining dams, called it "a shameful obfuscation."

"The narrow, contractually defined focus on 'proximate cause of failure only,' masks, avoids and obfuscates all questions of material relevance to a more comprehensive understanding," Ms. Bowker said.

"This could just stand in history as a big PR play on the part of Samarco, BHP and Vale, but it really depends on what the prosecutor does next," she added.

A BHP Billiton spokesman described the report as "another step in the process of finding out what happened."

"You need a proper technical understanding of what happened to draw effective lessons for what must change," the spokesman said. "This report provides the kind of detailed technical analysis the industry needs to improve dam management."

A Vale spokeswoman declined to comment.

Brazilian federal prosecutors filed a civil case against Vale, BHP Billiton and Samarco in May calling for 155 billion reais ($48 billion) in damages, comparing the fallout from Fundã o's collapse to that of BP's Deepwater Horizon oil spill in 2010 in the Gulf of Mexico.

The prosecutors also are working on separate criminal investigations into the deaths and environmental damages caused by the incident.

The fact that the report was commissioned by the mining companies, and the legal liabilities they now face, likely led its authors to stop short of making recommendations for how to make tailings dams safer in the future.

That's not always the case, however. A report published last year on the 2014 failure of Imperial Metals Corp.'s Mount Polley tailings dam in Canada included a "Where do we go from here?" section that included a series of recommendations for regulators, engineers and mining companies.

Parts of Dr. Morgenstern's presentation appeared to contradict earlier statements by Samarco, which previously characterized Fundã o's collapse as completely unforeseeable and possibly caused by the earthquakes.

A series of cracks in the dam's face, observed in August 2014, "were indicative of instability," Dr. Morgenstern said, noting that Samarco tried to reinforce Fundã o afterward. Some engineers say Samarco should have stopped depositing waste into Fundã o as soon as it detected the cracks, particularly considering the presence of communities and drinking-water supplies downstream of the dam.

Samarco also did not have instruments in place that could have detected slow-building deformities in the dam's shape that would otherwise have been invisible to the naked eye, Dr. Morgenstern said.

The earthquake, meanwhile, "was really quite small" and may only have "accelerated" the dam's collapse, he said. "The failure process without the earthquake was well advanced."

Write to Paul Kiernan at paul.kiernan@wsj.com

(END) Dow Jones Newswires

August 29, 2016 20:55 ET (00:55 GMT)

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