Search-and-rescue crews have confirmed a 12th death in the partial collapse of an apartment building in Surfside, Florida.

Miami-DadeCounty Mayor Daniella Levine Cava told reporters late Tuesday the crews are “continuing to make headway,” while 149 people remained unaccounted for.

About half of the 12-floor, 136-unitChamplain Towers Southcollapsed last Thursday, and since then rescue workers have slowly picked their way through the rubble using cranes, infrared scanners and dogs.

Levine Cava said the rescue crews were continuing “the brave and very, very difficult task, putting themselves in danger.”

“They have been working non-stop for nearly six days,” she said.

U.S. President Joe Biden and first lady Jill Biden are scheduled to visit Surfside on Thursday.

White House press secretary Jen Psaki told reportersthe Bidens want to thank those who have been “working tirelessly around the clock,” and meet with the families who have had to endure the difficult process of waiting for news about their loved ones. Psaki said the president will also talk with state and local officials to ensure they have the resources they need.

Levine Cava, in a statement Tuesday,welcomed the president’s upcoming visit and the efforts of federal and state agencies in response to what she called the “largest non-hurricane emergency operation” in Florida’s history.

Miami-Dade Office of Emergency Management Division Director Charles Cyrille told reporters late Tuesday more than 50 agencies were operating at the collapse site, including more than 900 personnel. (VOA)