The subway collision in Shaoxing, Zhejiang province, was caused by unauthorized access to the main railway line and negligence by a train controller, the provincial department of transportation said in an investigation report released Thursday.

The accident occurred at about 23:25 on Sept 13, when an empty Line 2 train returning to a depot for maintenance struck four cleaning workers on the track in Shaoxing. Three people were killed, and one was left critically injured.

It concluded that the immediate cause was that the cleaning personnel did not follow the designated route and entered the mainline without authorization. A train controller failed in their duties by not stopping the cleaning personnel or reporting the situation.

The provincial transport department classified the incident as a workplace safety accident, citing inadequate risk identification and safety controls by the operating company, as well as violations of safety procedures by personnel involved.