In this section, the Safety Council highlights what it believes establishes the most probable scenario of the in-flight breakup of CI611. The Safety Council concludes that the breakup was highly likely due to a structural failure in the aft lower lobe section of the accident aircraft, specifically in section 46. Because a large portion of section 46 wreckage was not found, the Safety Council can not draw a definitive conclusion of the source of the structural failure. However, the Safety Council believes that it is highly probable that the structural failure of the accident aircraft was initiated at S49L and STA 2100, where fatigue cracks were found during the detailed examination of wreckage piece item 640, which was related to the 1980 repair following a tail strike incident involving this aircraft. The support for this belief is examined in the subsequent sections