Chinese authorities say 13 people have been detained over the collapse of scaffolding at a power plant construction site that led to the deaths of 74 workers.

Most of the dead workers were building the interior concrete wall of a massive circular cooling tower 70 meters (230 feet) in the air when the scaffolding collapsed Wednesday morning.

The accident turned a construction site the size of a football field into a mangled heap of steel bars, concrete and wooden planks.

The focus of the investigation has turned to a major engineering firm, Hebei Yineng, which has taken on multiple high-profile power plant projects and has a history of workplace fatalities.

State media reports said the accident occurred during a change of work shifts, possibly accounting for the high death toll.