Another example was Pan Am Flight 103, which exploded over Lockerbie, Scotland, in 1988. It took a three-year investigation by British and U.S. authorities to determine that terrorists linked to Libyan intelligence planted a bomb on that aircraft.
It's important to note that it's still very early going with the EgyptAir investigation, and a search for answers could take years. It will take at least a month for Egypt to deliver its preliminary report on the crash, the head of the investigation team told that country's Al-Ahram newspaper.
Speculation almost inevitably will try to fill in the gaps in the meantime. Other possibilities pointed to by Stewart include a grass-roots terror group that wants to remain unknown, or an inside operator who wasn't on the plane but who had access to it. "They could be trying to protect that attacker," Stewart said.
Jeffrey Price, lead author of the textbook "Practical Aviation Security" and a professor at Metropolitan State University in Denver, agreed that a terrorist organization could have carried out the attack and then decided to remain silent. But he pointed out that in the case of Al Qaeda and ISIS, at least, those groups typically try to grab up media attention. "It's not typical of what they do," he said.