The city of Derna has buried thousands of people in mass graves, Libyan officials said on Thursday, as search teams scoured ruins left by devastating floods and the city’s mayor said that the death toll could triple or more. The deluge swept away entire families on Sunday night and exposed vulnerabilities in the oil-rich country that has been mired in conflict since 2011. Health officials have confirmed 5,100 deaths and say 9,000 people are still missing.

On Sunday night, residents said they heard loud explosions when the dams outside the city collapsed. Floodwaters washed down Wadi Derna, a valley that cuts through the city, crashing through buildings and washing people out to sea. The dams that collapsed outside Derna were built in the 1970s and have not been maintained for years, local media reported.