By Elliot Spagat
The Associated Press

AP Photo/Lenny Ignelzi A paramedic, police officer, and firefighter attend to an injured man at a construction site where a covered walkway collapsed, injuring at least 16 pedestrians in downtown San Diego Thursday.

SAN DIEGO — A block-long covered walkway next to a construction scaffold collapsed on Thursday, trapping and injuring 16 pedestrians, three critically.

About 25 people were on the walkway when its wooden walls and roof fell in, authorities said. Some scaffolding along the 3-story building also fell.

"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, including head trauma, when they were hit by falling debris, 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.

Carroll said his staff recognized some of the shelter clients as they sat on a nearby curb with neck braces. Others were bleeding lightly as they waited for an ambulance.

"They either had a meal here or live here and were walking to the trolley," he said.

Fire spokesman Daniel Calderon said investigators did not know what caused the collapse.

Julie Hattler, chief financial officer for Affirmed Housing Group, the building's developer, said her company hadn't determined the cause of the collapse.

"There's not much to say other than we have to find out what happened," she said.

The collapse occurred in downtown's East Village, the site of numerous construction and redevelopment projects.