Heightening the concern is US intelligence suggesting that terrorists have obtained sophisticated airport security equipment to test how to effectively conceal explosives in laptops and other electronic devices.
The intelligence, gathered in the last several months, played a significant role in the Trump administration's decision to prohibit travellers flying out of 10 airports in eight countries in the Middle East and Africa from carrying laptops and other large electronic devices aboard planes.
The findings may raise questions about whether the ban is broad enough. CNN has learned that, through a series of tests conducted late last year, the FBI determined the laptop bombs would be far more difficult for airport screeners to detect than previous versions terrorist groups have produced. The FBI testing focused on specific models of screening machines that are approved by the Transportation Security Administration and are used in the US and around the world.
"As a matter of policy, we do not publicly discuss specific intelligence information. However, evaluated intelligence indicates that terrorist groups continue to target commercial aviation, to include smuggling explosive devices in electronics," the Department of Homeland Security told CNN in a statement. "The U.S. government continually re-assesses existing intelligence and collects new intelligence. This allows DHS and TSA to constantly evaluate our aviation security processes and policies and make enhancements when they are deemed necessary to keep passengers safe. As always, all air travelers are subject to a robust security system that employs multiple layers of security, both seen and unseen."
US authorities have said the electronics ban is focused on the eight countries in part because of intelligence indicating a greater threat there. Intelligence and law enforcement assessments done in recent months also indicate that, though the broader vulnerabilities exist, the US has more confidence in detection machines and security screeners at airports in the US and Europe. Advanced technology and training helps mitigate the risk.
The US and European countries use a layered approach to security screening that goes beyond X-ray equipment, including the use of bomb-sniffing dogs and explosive-trace detection, according to US officials.
Aviation security expert Robert Liscouski, a former Homeland Security assistant secretary for infrastructure protection, said limiting the ban to eight countries makes sense based on the capability and locations of terrorist groups.
Not only are US and European airports better protected, he said, but developed countries have a "better policy regime" that allows them to set standards and ensure uniform compliance.
"We don't have the same level of confidence in other areas of the world because we don't have the government bodies and stature to assure compliance," said Liscouski, president of Secure Point Technologies.
When the electronics ban was announced, US officials told CNN they were concerned that terrorists had developed ways to hide explosives in battery compartments. But the new intelligence makes clear that the bomb-makers working for ISIS and other groups have become sophisticated enough to hide the explosives while ensuring a laptop would function long enough to get past screeners. Though advanced in design, FBI testing found that the laptops could be modified using common household tools.
FBI experts have tested variants of the laptop bombs using different battery and explosive configurations to assess how difficult it would be for airport screeners to detect them.
The intelligence that contributed to the ban on electronic devices was specific, credible and reliable, according to three officials who used the same words to describe it. One official called the intelligence "hair-raising."
At the same time, they also said there was no single, overwhelming piece of intelligence that led to the ban, rather it was an accumulation of intercepted material and "human intelligence."
The airline restriction, which took effect March 21, bans many electronics from the cabins of planes flying directly to the United States from airports in eight countries. Passengers on those flights must place electronic devices larger than cellphones in their checked luggage. Intelligence officials say it is more difficult for terrorists to detonate explosives remotely -- though it has been done -- and that placing devices in the cargo bay might reduce damage even if a bomb were to explode.
The United Kingdom, which possesses the same intelligence, implemented a similar rule this month for airplanes flying from six countries, including two that are not on the US list.
The ban was instigated after intelligence and law enforcement agencies determined that terrorists were working to place explosives inside laptop battery compartments in a way that would enable the devices to still power on in order to pass airport screenings, according to information shared with CNN.
Intelligence officials received a wake-up call in February 2016, when an operative from al-Shabaab, an al Qaeda affiliate in Somali, detonated a laptop bomb on a Daallo Airlines flight from Mogadishu to Djibouti. The explosives were hidden in a part of the laptop where bomb-makers had removed a DVD drive, according to investigators. Airport workers helped smuggle the bomb on the plane after it passed through an X-ray machine. In that case, the bomber was blown out of the airplane but the aircraft was able to make an emergency landing. However, experts have said the bomb would have been more devastating had the plane reached cruising altitude.
The military and intelligence community has grown increasingly concerned in the last few months about the potential ability of terror groups to get bombs on board airplanes, according to several US officials. The US has been tracking specific intelligence from al Qaeda in the Arabian Peninsula (AQAP), al Qaeda in Syria and ISIS, officials said.
The group with the greatest level of bomb-making expertise is al Qaeda in Yemen. Its master bomb-maker, Ibrahim al Asiri, has worked for years on designing explosive devices that can be hidden on bodies or in items such as printer cartridges. Since 2014, US officials have been concerned that Asiri's expertise had migrated to other groups.
To some degree, the fact that terrorist groups have been trying to install bombs in electronic devices is a testament to the success of advanced security techniques. Screening equipment and procedures have improved significantly since the terror attacks on September 11, 2001, making it more difficult to bring explosives onto airplanes. Bomb-makers continue to modify devices to get around enhanced screening.