One person is dead and another two remain unaccounted for after a seven-story parking garage under construction in Philadelphia partially collapsed, officials said.

A search and rescue operation is ongoing for the two individuals following Wednesday’s partial collapse, in what Philadelphia Mayor Cherelle Parker called a “complex process” due to the instability of the structure.

“We will not rest until everyone is accounted for from this tragedy,” Parker said at a press briefing late Wednesday.

Three people were rescued from the scene, according to Philadelphia Fire Commissioner Jeffrey Thompson. One was critically injured and died at a hospital, he said. The two others were treated and have since been released, he said.

The incident occurred around 2:17 p.m. Wednesday, according to Parker. A subcontractor was in the process of installing precast concrete floor decking and roof segments at the time, she said.

“After placement, a precast roof segment failed and fell to the level below, literally triggering a progressive collapse of connected sections across all seven levels,” Parker said.

Thompson said engineers and others are evaluating the building so that a safe search and rescue operation can occur.

“We will not stop until we have looked through every part of this affected structure to ensure that everyone is accounted for,” he said, noting that this process will “take time.”

“This building that has been impacted is unstable. It has to be secured,” he said. “We have to very carefully and methodically deconstruct this building for the safety of the people working on it, and ultimately, for the safety of the first responders who will continue the search.” 

The owner of the parking garage is the Children’s Hospital of Philadelphia, and the contractor for the project is based in Exton, Pennsylvania, according to Parker.

“We are prioritizing the safety of the construction workers at this time and working closely with the City of Philadelphia and our construction partners,” the Children’s Hospital of Philadelphia said in a statement.

Pennsylvania Gov. Josh Shapiro said his administration offers those on the ground “our full support as they continue rescue efforts at the scene and support the families impacted.”

Eight permits required for the construction were properly issued, and all inspections were up to date, according to Parker.

“The city will investigate this garage collapse thoroughly and efficiently to learn all of the relevant facts of this terrible tragedy,” she said.