One factor, a US official told CNN, was recent intelligence that the terrorist group al Qaeda in the Arabian Peninsula (AQAP) was perfecting techniques for hiding explosives in batteries and battery compartments of laptops and other commercial electronic devices. The intelligence was obtained in recent weeks and months, according to the official.
More generally, US officials have said intelligence shows that terrorist groups are still looking to target commercial flights by smuggling explosive devices in various items.
A US official tells CNN they are seeing a growing capability to target aviation from al Qaeda affiliates in Yemen, Syria, Somalia, as well as ISIS. It's not fully clear to what extent these groups are sharing specific information but "there is a growing pool of intelligence all pointing to threats to aviation" the official said.
ISIS is believed to be not as advanced in perfecting techniques for hiding explosives in electronics as AQAP, the official added.
If that were to change it could significantly increase the threat because of ISIS's still significant resources, global reach and remaining large cohort of Western recruits.
While the CIA believes AQAP and ISIS sometimes tactically cooperate
in Yemen, the senior leaderships of the two groups are at loggerheads, making technology transfers between them unlikely.
Drumbeat of al Qaeda plots
AQAP has for years been working to perfect techniques to get bombs on planes.
Between 2009 and 2012, AQAP master bombmaker Ibrahim al Asiri
orchestrated three terrorist plots to bring down American aviation.
In 2009, al Asiri fitted out a Nigerian AQAP recruit with an explosive underwear device containing PETN, a white powdery explosive that basic X-ray systems have difficulty detecting.
But the attempted attack failed to bring down the passenger jet, landing in Detroit on Christmas Day. Al Asiri used the same explosive compound concealed in printer cartridges in a plot to blow up cargo jets headed to the United States just before the 2010 mid-term elections.
And according to Western counter-terrorism officials, he used ETN, a closely related chemical compound, in a third plot targeting US aviation with a more advanced form of the underwear device. The plot was thwarted by a Saudi-British spy i
n 2012, who was recruited for the suicide attack and who retrieved the device.
The same year, Western intelligence agencies learned that AQAP was pioneering techniques to surgically implant devices inside potential bombers, according to British intelligence documents cited by the New York Times.
"(The Transportation Security Administration) took this threat seriously, especially because in 2009 al-Asiri had implanted a bomb in the rectum of his brother in an attack against then-Saudi Arabian counterterrorism chief Prince Muhammad bin Nayef," according to research published by the aviation security experts
Robert Liscouski and William McGann in CTC Sentinel
, the flagship publication of the Combating Terrorism Center at West Point.
They pointed out that in 2014 it emerged that al-Asiri and his team of bombmakers had "continued to do research and development on explosive devices, including shoe bombs." In the summer of 2015, al Asiri declared hitting the United States remained a priority.
In an interview with CTC Sentinel last September then CIA director John Brennan stated al Asiri was still at large and had become "very sophisticated in terms of his concealment capabilities."
In early to mid 2014, US intelligence agencies learned that the so-called Khorasan group -- a network of al Qaeda veteran operatives in Syria -- was developing plans to conceal bombs in personal electronics and smuggle them onto Western passenger aircraft.
This led to the Transportation Security Administration requiring tighter security measures at certain overseas airports with flights to the United States. As part of these measures, spot checks were introduced to make sure appliances taken on board could power up safely. That same year, US intelligence agencies came to the belief that AQAP was transferring bomb-making know-how
to the Khorasan Group.
In the past couple of years most of AQAP's energies appear to have been focused on taking advantage of the turmoil in Yemen to build up its position rather than plotting international terrorism. This expansion strategy has provided the group with more resources than ever before, including up to $100 million seized
from the al-Mukalla branch of the Yemeni central bank, creating concern the group could be providing extra funding to its bombmakers.
One former senior Western counter-terrorism official told CNN there is worry the recent escalation in US counter-terrorism operations in Yemen, including January's Navy Seal raid, might lead to the group retaliating.
The February 2016 a bomb attack on a Somali airliner leaving Mogadishu made clear terrorist groups' continued determination to bring down passenger jets.
The al Qaeda affiliated Somali terrorist group al Shabaab smuggled a laptop bomb onto a plane by recruiting two airport workers who handed the device to one of the group's operatives. The device was "sophisticated" and passed through an X-ray machine at the airport, a source close to the investigation told CNN.
The suicide bomber was blown out of the aircraft when the laptop bomb detonated and the plane was able to make an emergency landing. Only the fact that the aircraft had not reached cruising altitude prevented a disaster.
It is still not fully clear why the new restrictions are being implemented now, given the Somali attack was more than a year ago and the longstanding intelligence indicating al Qaeda groups have been working to conceal explosives in electronics for years.
Moreover, explosive detection experts believe it is much less likely that a laptop bomb would get through screening at international aviation hubs, like Dubai and Abu Dhabi, which operate state of the art detection systems and layered security.
Writing in CTC Sentinel,
Liscouski and McGann said that some media reports had falsely created the impression that al Qaeda was developing undetectable explosive devices. They stressed that the latest explosive trace detection technology when used in combination with the latest X-ray technologies were excellent at detecting the types of explosives being developed by AQAP, even if these explosives were concealed in the electronics of a laptop. Such systems are now in place at major modern airports like Dubai and Abu Dhabi.
Another aspect that is puzzling counterterrorism analysts is why the ban on laptops applies only to cabin luggage when the same explosive detection technology is used for both hand and checked baggage.
This might suggest Western intelligence is concerned about a threat stream involving manually detonated devices, like in the Christmas 2009 underwear bombing attempt.
Aviation security experts tell CNN there is particular concern over carry-on explosives because suicide bombers have sought to locate themselves to do maximum damage to the fuselage, increasing the chances of a catastrophe.
In the attempt to blow up the Somali airliner, the bomber knew precisely were to sit to maximize damage, a source close to the investigation told CNN. When it comes to hold luggage, terrorists have no control over the placement of their baggage creating the possibility a bomb hidden inside would be some distance from the fuselage and be insulated by other luggage, the aviation security sources told CNN.
Another reason why terrorists might want to bring explosive devices into the cabin is that their plans might require them to assemble them mid flight from multiple components possibly carried by multiple passengers. A former senior US official told CNN this has also been a longstanding concern of US security agencies.
Timer triggered devices are however well within the skill set of a group like AQAP.
US officials believe a timer was used
to trigger the bomb which brought down a Russian passenger jet leaving Sharm el Sheikh, Egypt in October 2015. Western intelligence services believe ISIS's Sinai affiliate carried out that attack by recruiting an airport insider
who placed the device inside the aircraft.