A construction scaffold that spanned a city block and a wooden walkway next to it collapsed on Thursday, injuring 16 pedestrians. Injuries to three of the injured were life-threatening, authorities said. About 25 people were walking in the wooden walkway next to the three-story building under construction when the scaffold fell. The roof and a side of the walkway 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. Ambulances and fire trucks converged on the scene, in San Diego's downtown East Village neighborhood. Investigators did not know what caused the collapse, fire spokesman Daniel Calderon said.