Inadequate cofferdams, illegal construction and poor supervision resulted in the bridge pier collapse which killed 11 people in Southwest China's Chongqing Municipality in mid-October, local authorities said on Thursday.

A pier of an under-construction bridge across the Yangtze River in Fengdu County collapsed and became submerged in water on October 12, leaving 10 workers and a local resident dead and two other people injured.

Investigations have placed the blame for the accident on the inadequate and unstable structure of steel cofferdams at the construction site, severe defects in the project procedures and regulation violations during the project supervision process, according to sources with the work safety committee under the Chongqing Municipal government.

Further investigation is under way. The people responsible for the accident will be punished, the sources said.