Complex sentence: On examination of the wreckage investigators discovered that in addition to the external ones, internal doublers had also been affixed to the root area of both wings, although maintenance records for these repairs were not available.
Split into simpler sentences: On examination of the wreckage, investigators discovered that, in addition to the external doublers, internal doublers had also been affixed to the root area of both wings. However, maintenance records for these repairs were not available.