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.