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BHP said Cleary Gottlieb would draw on expertise in disciplines such as geotechnical engineering and hydrology.
The company reiterated its pledge to publicly release the findings.
There are 17 confirmed fatalities, comprising 12 Samarco workers and five community members, while a further two workers remain unaccounted for.
BHP appointed Flavio Bulcao as country director, Brazil, as its in-country representative reporting to chief commercial officer Dean Dalla Valle, who is leading the Samarco response.
BHP said it was still yet to receive formal notice of proceedings reportedly initiated by the Brazilian government against Samarco, which demanded it establish a 20 billion real ($US5.2 billion) rehabilitation fund.
On Friday, the Federal Court of Belo Horizonte ordered Samarco to deposit 2 billion real into a court-managed bank account within 30 days to pay for the initial clean-up, with a daily fine of 1.5 million real payable for non-compliance.
The court also prohibited BHP and Vale from transferring or selling Samarco tenements, and ordered that certain remediation work be completed within specific timeframes.
Remediation work completed by Samarco and its consultants is broadly in line with court requirements.
The relocation and accommodation of the more than 600 people left homeless continues, with all affected given the opportunity to move into temporary accommodation before Christmas Day.
Interim work to repair the damaged dams is more than 50% complete.
BHP also confirmed that a report issued by the Brazilian Geological Service and National Water Agency last week supported earlier claims that the tailings material discharged from the dams was non-toxic.
