Usually, when it's a terror attack, they've claimed responsibility by now.
No terror groups have attempted to take credit for the crash of EgyptAir Flight 804, on the fifth day after the plane and 66 people on board disappeared over the Mediterranean. That may mean that something besides terrorists caused the tragedy; it could also mean something more sinister.
The most obvious explanation for the silence from jihadists and other terror groups is that they didn't do it. "The big possibility is that it was just an accident, and something happened that took the plane down quickly, probably incapacitating the pilots and making it impossible for them to call in a mayday," said Scott Stewart, vice president of tactical analysis at research firm Stratfor.
But there's a possibility — and historical precedent for the idea — that no terror group has claimed the destruction of the flight from Paris to Cairo because it either wants to protect operatives it has established inside transportation networks, or it has developed new techniques that it wants to replicate to more devastating effect later.
It's happened before.
"The bombing of Philippines Airlines Flight 434 in December 1994 was not claimed because the planners hoped to use an improved version of the same device in a larger attack targeting 10 trans-Pacific airliners," Stratfor said in a brief over the weekend.
In that attack, known as the "Bojinka plot," terrorists led by 1993 World Trade Center bomber Ramzi Yousef tested the new idea of using chemicals to blow up airliners. The plan was to follow up on Philippines Flight 434 with simultaneous attacks on several aircraft over the ocean.
Since that time, airports around the world have countered any further chemical plots by screening the liquids that passengers bring on board.
"If they have something that's new, they don't want to reveal that until they pull off something spectacular," Stewart said.
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.
Gen. Assad Hamdy, former deputy director of Egyptian intelligence, told NBC News that a terrorist attack is the most likely cause of the crash.
Other officials from Egypt very quickly blamed terrorists for the attack, and U.S. intelligence sources have hinted at a terrorist attack as well, though experts have generally walked back that position since it was revealed that smoke was detected on EgyptAir 804 before it went down.
The investigation into the EgyptAir crash is especially difficult given that the plane disappeared over deep water. The search for Malayasia Airlines Flight 370, which went down more than two years ago, has only recently yielded parts of its wreckage.
The mystery of what brought down Air France Flight 447, which crashed over the Atlantic in 2009, remained unresolved for three years. That plane was lost for reasons linked to a technical failure.