At least five people have died after a building with more than 20 stories collapsed in an upmarket area of Nigeria’s economic hub, Lagos, on Monday, authorities said.

The building, located on Gerald Road in the city’s affluent Ikoyi neighborhood, is a construction site for luxury apartments. It is unclear how many were inside when it fell, but people were feared to be trapped.

Search and rescue operations from multiple agencies continued through the night and into Tuesday morning. The Lagos State Emergency Management Agency (LASEMA) confirmed the five deaths in a Facebook post on Tuesday. It did not say how many people are currently missing.

“I thought it was an earthquake when I rushed out of my apartment just after 3 p.m. I felt the building move and knew something was wrong,” Olu Apata, a nearby resident and the president of the Nigerian Bar Association Said.

Apata said the building has been under construction for the past two years and that the developer was at the site meeting with prospective buyers earlier on Monday.

Three people have since been rescued from the collapse, according to a statement from the Lagos State government, which added that the city’s rescue unit, fire service, and ambulance service were on the scene.

The investigation into the cause of the collapse is ongoing, and officials on the ground are evaluating possible damage to the surrounding structures, the statement said.

Muhammad, a civil servant said he was in his office next door when the high rise came down. “We just heard an unusually loud sound from our office building and the building we’re occupying was vibrating. So, I looked through the window, only to see the building collapsing floor after floor. And of course, we all ran to safety.”

It took a while for rescue services to arrive at the scene. Hours after the collapse, hundreds of people were milling around the site, helplessly surveying slabs of concrete piled high in a heap. Locals were digging through the wreckage by hand.

Some in the crowd were furious that an official emergency response had not started.

Instead, rescue efforts were being carried out by passersby such as Rashid Olamilekan, who said he had pulled three construction workers from the rubble. “I am a Nigerian. I have to rescue my people,” he said. The rescued workers had been taken to the hospital, Olamilekan added.

The Nigerian Red Cross was also there.

Building collapses are not uncommon occurrences in Lagos, Nigeria’s largest city with a population of around 20 million people.