The government has assessed that unauthorized structural changes and poor concrete were the sources of an apartment collapse in the southwestern city of Gwangju that killed six people in January.

A committee under the transportation ministry tasked with investigating the collapse disclosed the evaluation in a news conference on Monday.

The probe committee said the construction of the 39th floor and the apartment’s supporting system were altered from the initial blueprints without due notice.

The committee added that most of the concrete used in the apartment construction fell below the specified concrete strength of 85 percent. The committee’s chief, Kim Kyu-yong, said he believes more water was added in the process of mixing and forming concrete.

The committee suspects that the insufficient concrete strength compromised the safety of the apartment building.

The committee also found that the construction supervisor had failed to cooperate with related engineers when altering the initial blueprints.