A stairwell roof at a parking garage being built in Philadelphia collapsed suddenly, killing one person, while two others are missing and presumed dead, authorities said.

The section of roof at the Children’s Hospital of Philadelphia’s garage fell Wednesday, triggering a “progressive collapse of connected sections across all seven levels,” Mayor Cherelle Parker told reporters.

“Let me be very clear about something at this moment: We are not, we will not give up on these individuals and we will not rest until everyone is accounted for from this tragedy,” she said.

By Thursday, she said search dogs hadn’t found any signs of life at the collapse site.

Crews had initially rescued three people, including one who was critically injured and later died at the hospital, said Philadelphia Fire Commissioner Jeffrey Thompson. Two others were treated and released.

The building is unstable, so crews need to take it apart in order to search the entire structure, Thompson said.

“We’re actually going to deconstruct and de-layer this building, and that will make it safe for my members to get inside and get down to the lowest levels to ensure that we don’t have any survivors down there,” he said.

All of the project’s required permits were properly issued and inspections were up to date, according to Parker. She said the city will investigate the collapse.

The hospital said in a statement that it is prioritizing the construction workers’ safety and working closely with the city and its construction partners.