June 6, 2016 7:13 AM EDT

Perhaps clarity on the May 19 crash of EgyptAir Flight 804 will surface with the aircraft’s black box, which sensors have located on the Mediterranean Sea floor. But weeks after officials from Cairo to Washington speculated that the plane went down at terrorists’ hands, no terrorist group has taken responsibility.

That’s unsettling all by itself. ISIS and al-Qaeda tend to make their claims promptly. And even when they don’t, specious claims bubble up from obscure groups trying to seize the spotlight. This time: nothing. In the vacuum, terrorism specialists are weighing the possibilities, few of which are reassuring:

1. It wasn’t a terrorist attack. No claim may well mean no bomb. “Why bring down a plane if you’re not going to take credit for it?” asks Clint Watts, former executive officer at West Point’s Combating Terrorism Center. He finds “somewhat reassuring” the plane’s departure from Paris’ Charles de Gaulle, an extremely security-conscious airport, and how late into the four-hour flight it went down, over international waters. As much as the absence of suspicious names on the passenger list, those facts argue against the presence of a suicide bomber, because usually, as Watts says, “you get in the plane, you reach altitude, you detonate it.”

2. It was a bomb, but smuggled aboard by means the terrorists hope to use again. The private intelligence firm Stratfor dubs this the “more sinister,” if less likely, explanation. It’s happened before: in December 1994, Ramzi Yousef, the mastermind of the 1993 World Trade Center bombing, tucked a homemade bomb with a timer into the life vest under his seat on a Philippine Airlines flight, then left the plane. When it detonated on the next leg of the flight, Yousef made no public claim of responsibility—because the bomb was a test run for planting explosives in the same way aboard 11 U.S. airliners crossing the Pacific. The plot was discovered when a bombmaker accidentally set fire to a Manila apartment. That discovery led to new security warnings instead of disaster. “In a worst-case scenario,” Stratfor cautioned in a briefing about EgyptAir Flight 804, “we may have a competent bombmaker on the loose with knowledge of how to get a bomb onto a plane, and the authorities have no idea what method he is using.”

3. Some new group did it, and wants to lay low. The absence of claims from ISIS and al-Qaeda “makes me think it might be a new group, or a splinter of a group,” says Jonathan Schanzer, a former U.S. Treasury terrorism analyst. The likeliest group would be “an indigenous organization, maybe in Egypt,” he says. Targeting EgyptAir, a parastatal company closely identified with the government, amounts to striking a blow against President Abdul Fattah al-Sisi, who alienated millions of Egyptians when he deposed his elected Islamist predecessor and outlawed the Muslim Brother­hood. Schanzer notes that a similar crackdown on Islamists in the 1990s energized both the Islamic Group and the Egyptian Islamic Jihad (which later merged with al-Qaeda). Now, as the Brotherhood has been forced underground, history may be repeating itself. Samuel Tadros, a Hudson Institute analyst, says numerous small groups have sprung up from the Brotherhood to answer Egyptian-state crackdowns, but “they have specialized in low-level violence,” like throwing Molotov cocktails at police and toppling electrical towers. Taking down a plane would be a huge leap, he says.

4. Finally, it might even have been a lone wolf, whose presence among the 66 dead would at least explain the absence of a claim. The hope is that the black box will reveal data or pilot conversation that shifts speculation further into the realm of fact. “It’s just very odd,” Schanzer says. “Every day that goes by, it conjures up more thoughts—and scary thoughts.”

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