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Warning: This story contains disturbing images and video footage of human fatality. Viewer discretion is advised.
Boaters can be heard screaming in videos of the incident as the rock formation cascaded on the boats in Furnas Lake, which is located in Minas Gerais state.
One of the videos shows the moment right before disaster struck, with several people heard yelling of stones falling and urging the other boats to steer away from the rock face.
Over 30 others were also injured, including nine who were hospitalised.
At least 24 people survived on the two boats that were directly hit. The search concluded on Sunday, 9 January, with all victims and survivors accounted for.
Authorities are investigating the cause of the mountain collapse but suggested it could be due to days of heavy rains in the area, which have caused flooding in the state and displaced thousands of people.
Tourists flock to see the cliffs, caverns, and waterfalls at Furnas Lake, a popular picturesque destination during the summertime.