In a series of public statements, government officials in London and Washington blamed the new restrictions on a terrorist plot to blow up transatlantic airliners, which has led to at least 24 arrests.
"The terrorists' aim was to smuggle explosives onto aeroplanes in hand luggage and to detonate these in flight," Paul Stephenson, Scotland Yard's deputy commissioner, told reporters.
Homeland Security Secretary Michael Chertoff added later in the day that the alleged plotters "planned to carry the components of the bombs, including liquid explosive ingredients and detonating devices, disguised as beverages, electronic devices or other common objects."
Liquid and gel explosives are hardly new, of course. Inventor and industrialist Alfred Nobel began manufacturing nitroglycerin in 1865 in the suburbs of Stockholm, Sweden, calling the explosive mixture by the brand name "blasting oil." Later, Nobel found that if nitroglycerin were diluted with nitrocellulose, it became a more stable, glutinous substance he dubbed "blasting gelatine."
Terrorists have used liquid explosives before, with mixed results.
Ramzi Yousef, who was involved in the 1993 World Trade Center bombing, concocted a plan a year later to attack 11 flights traveling from central Asia to the United States. The plot was uncovered in the Philippines in January 1995, two weeks before its execution date, after Yousef and others accidentally started a fire in their apartment and police showed up.
Before he was arrested, Yousef did a trial run with a lower-power bomb. He assembled it in the lavatory of a flight from Manila to Japan and left it on board after he departed on a connecting flight. The bomb exploded, but the Boeing 747 limped to an emergency landing with only one casualty.
Documents found on Yousef's computer that emerged during his trial (Click here for PDF) showed that the plotters had filled bottles of contact lens solution with nitroglycerin and planned to use Casio digital watches as the timers, coupled with two 9-volt batteries in the bomb as a power source. The 9/11 Commission's report said Yousef also had prepared dolls wearing clothes containing nitrocellulose, an explosive compound.
That kind of stealth explosive seems to be what provoked the dramatic reactions by Homeland Security and other officials on Thursday. Some reports said the plotters would conceal their peroxide-based explosives in a sports drink and detonate it with a disposable camera's flash. Others said, however, that the bomb would be "detonated by using heat or friction."
"Travelers are going to be inconvenienced as a result of the steps we've taken," President Bush said while traveling in Wisconsin. "I urge their patience and ask them to be vigilant. The inconvenience...occurs because we will take the steps necessary to protect the American people."
Air travelers flying inside the U.S. are still permitted to bring laptops and electronic devices as carry-on items, though both U.K. and U.S. passengers were generally prohibited from bringing liquids or gels into the passenger cabin.
Nitroglycerin may be one of the easier liquid explosives to create in a rudimentary laboratory, but it's not the only one that could be employed by bomb makers. Other candidates are nitromethane (sometimes used as a cleaning solvent), dithekite, nitroethane, and methyl nitrate (derived from nitric acid). Fixor is a commercial two-component explosive, based on a flammable liquid that's designed to replace plastic explosives, but one which requires a detonator cap.
Saboteurs are believed to have used liquid explosives smuggled on board in a bottle of alcohol to attack Korean Air flight 858 in 1987. The bomb, apparently left on board by passengers who deplaned, killed 115 people and has been attributed to North Korean agents.
Because conventional X-ray machines used at airport security checkpoints can't reliably differentiate between innocuous beverages and liquid explosives, the explosives are difficult to detect.
A report last year from Congress' research arm says that chemical traces often can be detected through screening devices at airports that use puffs of air to dislodge debris, but warned that the "portals" already in use at some airports are expensive and slow.
In addition, the report said, "novel explosive materials will probably not be detected by these systems." Also, if a bomber takes proper precautions, such as carefully sealing containers and not wearing contaminated clothes, those screening devices may not help.