The liquid explosives suspected of being at the centre of the alleged aircraft bomb plot could be tricky for terrorists to handle but difficult for checks to detect, bomb and security experts said on Thursday.

The raw materials to make liquid bombs that required no detonator were readily available but the mixture often had a high risk of going off prematurely, people familiar with explosives said.

The plot alleged by the British and US authorities has some superficial similarities to the case of Ramzi Yousef, who was convicted in 1996 of planning to blow up a dozen US aircraft, possibly with bombs made of liquid nitroglycerine.

Mr Yousef, who developed a low-cost nitroglycerine-based bomb, virtually impossible to detect with airport technology, was an innovator in the art of destruction, according to terrorist experts.

The 1994 “test” bombing, in which a Philippine Airlines passenger was killed and 10 were wounded, was just a small part of his blueprint for killing thousands of Americans, prosecutors said during his trial.

John Wyatt, a former
counter-terrorism official who is now a security adviser to governments and large companies, said the advantage of being able to buy the parts of a liquid bomb had to be weighed against the problem that even a small shock could detonate it prematurely.

“Actually doing it, putting it together, carrying something as volatile as a liquid explosive, is another matter,” he said. “[But] even if the device went off in the process in an airport, they will have achieved some sort of success.”

Michael Chertoff, US homeland security secretary, said the plot involved smuggling explosive liquids disguised as drinks on board, along with detonating devices. He said passengers would now be prohibited from taking liquids, including drinks and hair products, on board aircraft.

Explosives experts said many of the liquids used in bomb-making were hard to detect because they were colourless or could easily be treated and passed off, for example, as milk in a baby’s bottle. They would probably be missed by a “superficial search”, said one expert.

Hans Michels, a professor of safety engineering at Imperial College, London, said the bombs would either have been prepared and detonated by a shock, mixed on board and set off the same way, or pre-mixed and triggered by a detonator.

Each method had its advantages and disadvantages, he said. Shock-
triggered bombs could go off too early, detonators could be detected by security checks and concoctions mixed in situ could attract attention because they involved substances, such as nitric acid, that give off fumes.

Depending on the type of bomb and where it was placed, the effect could blow a hole in the aircraft fuselage or spread gobbets of burning gel through the cabin, Prof Michels said.

“My own instinct is that it is likely to be something that blows a hole,” he said. “I think very, very quick blasts are what they are really after.”

Experts said one example of a liquid bomb that could be made from readily avail-able materials was dithekite, a mixture of nitric acid, water and nitrobenzene. Other possibilities included bombs based on nitroglycerine or nitromethane, which is used in the oil industry to clear debris from wells, Prof Michels said.

Mr Wyatt said it was possible that police had exposed the alleged plot by tracking thefts or unusually large purchases of materials used in liquid bombs.

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