A covered walkway that spanned a city block next to a construction scaffold collapsed Thursday, injuring 16 pedestrians, three critically.
About 25 people were on the walkway when its wooden walls and roof fell in, authorities said.
"The walls started moving, then the bang. Everything started coming down. Everyone started screaming," said Abigail Reckermann, 50, who went to the hospital with a swollen ankle.
Ariel Medina, 34, was uninjured but saw a board fall onto the back of a man she was talking with.
"It was a living nightmare," Medina said. "The whole thing just caved in. People were trapped."
Three people suffered life-threatening injuries, authorities said.
Father Joe Carroll, president of St. Vincent de Paul Village, said many of the injured had just eaten lunch or were living at the homeless shelter he runs across the street. The shelter had served lunch to about 1,000 people.
Fire spokesman Daniel Calderon said investigators did not know what caused the collapse.
A sign seen in the debris identified Allgire General Contractors in Carlsbad as the builder. A woman who took a call at Allgire's offices said she could not comment then hung up the phone.
Ambulances and fire trucks converged on the scene.
The collapse occurred in downtown's East Village, the site of numerous construction and redevelopment projects.