NEW ORLEANS — A cascade of metal and debris rained onto a New Orleans street on Saturday as portions of a hotel under construction collapsed, killing two people and setting off a precarious rescue effort inside the dangerously unstable building, the authorities said.

The collapse, which was reported at around 9 a.m., prompted the authorities to evacuate buildings surrounding the hotel and cordon off an area, near the French Quarter, that is often packed with tourists. Officials warned on Saturday that the building was still “very unstable,” and could collapse further. A 270-foot crane at the site was also at risk of falling.

Investigators examined the building and talked to workers who had been inside before sending in rescuers to try to find the missing. Rescue workers entered several hours after the collapse, the authorities said. Mayor LaToya Cantrell of New Orleans said late Saturday that a second person had died, and a third person remained unaccounted for.

“Remember, we have first responders going into a very dangerous building right now, trying to do their job,” Collin Arnold, the city’s director of homeland security and emergency preparedness, told reporters.