SAN DIEGO (AP) — A construction scaffold that spanned a city block and a wooden walkway next to it collapsed on Thursday, injuring 16 pedestrians.

About 25 people were walking in the wooden walkway next to the three-story building under construction when the scaffold fell. The roof and 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.

Injuries to three of the injured were life-threatening, authorities said.

Ambulances and fire trucks converged on the scene, in downtown's East Village neighborhood.

Investigators did not know what caused the collapse, fire spokesman Daniel Calderon said.