One person is dead and two are missing after a parking garage collapsed in Philadelphia.

A roof segment fell and triggered a “progressive collapse of connected sections across all seven levels,” Mayor Cherelle Parker told reporters Wednesday.

“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.

Crews 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.

He said the building is unstable and so they need to take it apart in order to search the entire structure.

“We’re actually going to deconstruct and delayer 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.

The parking garage is owned by the Children’s Hospital of Philadelphia and all 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.

Pennsylvania Gov. Josh Shapiro said his office is supporting the rescue effort.