At least 12 construction workers died and four are missing after a steel cable snapped during a tensioning operation on an under-construction railway bridge in Qinghai, northwest China, around 3 a.m. Friday. Sixteen workers were on the bridge at the time.

Aerial photos show a large section missing from the bridge’s curved arch, with part of the deck hanging into the Yellow River.

Teams are rushing to the scene using boats, aerial drones, and robotic devices to carry out the rescue. The 1.6-kilometer-long bridge stands 55 meters above the river.