Officials have called off rescue efforts at the 12-story Champlain Towers South in Surfside, Florida over concerns that the portion of the complex still standing is structurally unstable, endangering workers sifting through the rubble pile, CBS News reports.

The stoppage likely signals a change in the status of the efforts of crews on site, switching from a rescue effort to a recovery effort, to find bodies rather than survivors.  So far, 18 people have been confirmed dead as a result of the June 24th collapse.  More than 140 people remain unaccounted for.

Engineers measured a 6-12 inch shift in a large concrete column that looms of the collapsed section of the complex where workers have been digging through rubble looking for survivors.  Should that column collapse, it could bring down the remaining standing structure. 

President Joe Biden and First Lady Dr. Jill Biden are visiting the site Thursday to offer support to the families of victims and rescue workers.

During a Thursday press conference, Miami-Dade County Fire Chief Alan Cominsky related a heart-wrenching account of hearing a woman’s voice emanating from the rubble in the hours after the collapse.  Although rescues tried to find her location, her voice quieted in the hours they were searching and hasn’t been heard since.

“Eventually we didn’t hear her voice anymore, we continued to search,” he said. “Again… that’s emphasizing the magnitude of what we’re going through.”