Two days after the crash of a Metrojet Airbus A321-200 an early focus of the investigation is shifting to why the aircraft apparently broke up in mid-air.

Flight 7K9628, an international charter service from Sharm El Sheikh, Egypt to St. Petersburg, Russia had taken off at 5.49 a.m. on Saturday and crashed into a remote region of the Sinai peninsula killing all 224 on board. The fact that contact with the aircraft was lost at above 28,000 ft. and that debris is scattered across a large area leads investigators to believe that the aircraft broke up at altitude and not upon impact. According to tracking website Flightradar24, the aircraft showed large fluctuations in speed and altitude in the final moments of flight.

There has been a claim by the local branch of the Islamic State that it has brought down the aircraft, but security experts doubt rebels in the region would have the equipment to shoot down an aircraft at high altitude. Traces of explosives should be recognizable in the debris if indeed the aircraft had been shot down by a missile, as in the case of Malaysia Airlines flight 17 over Ukraine in 2014, or brought down by a bomb explosion.

Debris analysis will also be key for efforts to determine whether a structural failure of the fuselage occurred because of a maintenance issue or another reason. The aircraft, last registered as EI-ETJ (and on lease from ILFC to Kogalymavia, operating under the Metrojet brand) had been involved in a tail strike on November 1, 2001, when it flew as F-OHMP for Lebanese carrier Middle East Airlines (MEA). In what looked like an unstable approach to Cairo International Airport’s runway 5R, the aircraft suffered severe rear fuselage damage upon landing. The aircraft was subsequently repaired and returned to service in early 2002, no details about the repair have become public so far.

Inflight break-ups because of structural failure are extremely rare, but have happened in the past. On May 25, 2002, a China Airlines Boeing 747-200 (registered B-18255) flew a scheduled service from Taipeh to Hong Kong and disappeared from radar during cruise flight at 35,000 ft. The accident report concluded that the crash was highly likely to have been caused by a structural failure in the rear lower fuselage that could be traced back to a February 7, 1980, tail strike in Hong Kong after which the aircraft had not been properly repaired.

On April 28, 1988, an Aloha Airlines Boeing 737-200 lost part of the upper fuselage on a flight from Hilo to Honolulu. It diverted and landed in Maui. The accident investigation concluded that corrosion had not been detected and caused the failure of a lap joint.

The Metrojet investigation got off to a chaotic start with Egyptian officials from various government departments as well as Russian state representatives present at the crash scene sending conflicting messages and wrangling over where the flight data recorder and cockpit voice recorder (both of which have been recovered by now) should be examined. According to ICAO Annex 13, Egypt should be leading the investigation, but other affected parties are also part of it. Egypt can delegate the lead in the process to another party if it so wishes.