The Libyan city of Derna has buried thousands of people in mass graves, officials said Thursday, as search teams scoured ruins left by devastating floods and the city’s mayor said the death toll could triple.

The deluge swept away entire families on Sunday night and exposed vulnerabilities in the oil-rich country that has been mired in conflict since a 2011 uprising that toppled long-ruling dictator Moammar Gadhafi. Health officials have confirmed 5,500 deaths and say 9,000 people are still missing.

Daniel, an unusually strong Mediterranean storm, caused deadly flooding in towns across eastern Libya, but the worst-hit was Derna. As the storm pounded the coast Sunday night, residents said they heard loud explosions when two 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.

A U.N. official said Thursday that most casualties could have been avoided. “If there would have been a normal operating meteorological service, they could have issued the warnings,” World Meteorological Organization head Petteri Taalas told reporters in Geneva. “The emergency management authorities would have been able to carry out the evacuation.”

The WMO said earlier this week that the National Meteorological Center had issued warnings 72 hours before the flooding, notifying all governmental authorities by email and through media.

Officials in eastern Libya warned the public about the coming storm but did not suggest the dams could collapse or present an evacuation plan.

Libya’s Tripoli-based Prime Minister Abdul Hamid Dbeibah on Thursday acknowledged issues in the maintenance of two dams that burst, causing devastating floods that killed over 5,000 people.