Two dams burst upstream from the coastal city, releasing a wall of water that swept away buildings and the people inside them.
Nearly 10,000 people are missing in Libya after a severe storm slammed into its eastern coast this week, according to the Red Cross.
The death toll stands at 6,000 and is expected to rise as operations continue to recover bodies.
“The death toll is huge and might reach thousands,” said Tamer Ramadan of the International Federation of Red Cross and Red Crescent Societies.
Why was the storm so deadly?
Storm Daniel formed over Greece on September 4. It caused strong winds, heavy rains, flooding and deaths there and in Turkey and Bulgaria before crossing the Mediterranean.
The storm made landfall in Libya a few days later, causing flooding on Sunday in cities along its eastern coast, including Benghazi, Bayda and Derna.
Dams collapse outside Derna
Derna was the hardest hit after two dams burst upstream from the city, releasing an estimated 30 million cubic metres (39 million cubic yards) of water that tore through the city of about 100,000 inhabitants.
The water raced down Wadi Derna from the mountains to the sea. Deputy Mayor Ahmed Madroud said the way the city was built put most of the population in the water’s direct path.
The dams had not been maintained for more than two decades, and the infrastructure was not built to withstand the effects of this week’s floods, Madroud said.
Libya has seen more than a decade of conflict and is politically divided. An internationally recognised government sits in Tripoli in the west, but Derna is in an area controlled by a rival administration based in Benghazi.
Satellite images show scale of destruction
Front-line workers are still combing through the flood debris.
With no telephone or internet service, the city was cut off from the rest of the country, and help began to arrive 36 hours after the devastation. The floods damaged or destroyed many roads, making it harder to access those who needed help.
Madroud told Al Jazeera, “At least 20 percent of the city has been destroyed.”
Drag to the left to see what remains of Derna [PlanetLabs]