Complex sentence: On 19 January, investigators released a statement based upon the testimony of the survivors, which stated that an open flame cooker used as a heating device was considered the likely source of the ignition, as the bus itself did not have a functional heater.
Split into simpler sentences: On 19 January, investigators released a statement which was based upon the testimony of the survivors. This stated that an open flame cooker used as a heating device was considered the likely source of the ignition--the bus itself did not have a functional heater.