Mining company BHP and its investment partner, Brazilian giant Vale, have serious questions to answer about what they knew about the breached tailings dam built by the companies’ co-owned mining company Samarco in the Brazilian province of Minas Gerais. The breach last week killed at least six people, and another 21 remain missing. Beyond a death toll that is likely to climb, compensation and remediation costs are likely to be significant.

For several days, Brazilian media have been discussing a 2013 report for the Minas Gerais environment ministry about safety concerns over the dam, and this report has now received media attention in Australia. The report is a key reason why a state government prosecutor has said the breach was no accident.

Neither Samarco nor BHP nor Vale have admitted knowing about the report, which appears to warn of the exact scenario that led to the dam breach. But the report is publicly available from the ministry’s website. It beggars belief that Samarco was not aware of the report, and as co-owners of Samarco, BHP and Vale must surely have been aware of it as well, or face questions about why they didn’t.

In recent days, both Vale’s and BHP’s share prices have been hit hard. Both companies must explain to investors, regulators and above all the communities affected by this tragedy what they knew about the report, and when they knew it. The companies’ joint commitment, announced this morning, to an emergency fund is welcome. But there are serious questions about responsibility for this disaster: if Samarco, and BHP and Vale, were warned about the safety of the dam two years ago, then the most serious consequences should follow.