Engineers have said a 21-storey building that collapsed in Nigeria’s commercial capital of Lagos early this month was originally designed only for six floors before more were added to the structure.

The high-rise structure was still under construction in the upscale Ikoyi district of the city when it crumbled on November 1, trapping dozens of site workers and others inside.

At least 45 people, including the owner of the building, were killed, while 15 others were pulled out alive from the debris, according to the latest tally by the state government.

Although the government has set up a panel to determine the cause of the collapse within one month, the Nigerian Institution of Structural Engineers (NIStructE) said variations on the original design were to blame.

“There are clear indications of several design brief changes on the project and the engineering and management of these changes appear to have been seriously inadequate,” NIStructE President Kehinde Osifala said in a statement late on Tuesday.

“The building that collapsed was initially designed for just six floors, and later to 12 floors, before this was further changed to 15 floors,” he said.