Freight flights are suspended and toner cartridges banned following the cargo terror plot. Experts tell Channel 4 News the devices would have detonated if intelligence had not foiled the attack.

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Home Secretary Theresa May said flights containing unaccompanied freight from Somalia and Yemen are suspended in a "precautionary measure" based on "possible contact between al-Qaeda in Yemen and terrorist groups in Somalia".

Ms May told MPs the emergency measures come into force from midnight.

Toner cartridges larger than 500g (17.6oz) will also be banned from hand baggage on flights departing from the UK and also on cargo flights unless they originate from a regular shipper with security arrangements approved by the Department for Transport, she said.

The two devices discovered on cargo planes at the weekend contained 300g and 400g of PETN - pentaerythritol tetranitrate - a powerful, odourless explosive which can be difficult to detect using conventional security.

PETN is a common explosive and can be used to make SEMTEX. The explosive, a fine white powder that when dry resembles sugar or salt, is part of the same chemical family as nitroglycerin.

Although it can be difficult to obtain legally in its pure form, experts have told Channel 4 News it is the explosive of choice for extremists.

It is not the first time PETN has been used in a terror attack.

Richard Reid, the "shoe bomber", tried to set off a PETN device on an American Airlines jet to Miami in 2001.

Weapon of choice
Explosives expert Prof Hans Michels tells Channel 4 News why PETN is regularly used in bomb attacks.

- Explosive material PETN is about 35 per cent more powerful than TNT
- It is colourless as crystal and not readily 'seen'
- It is relatively insensitive to occasional shock or friction (far more so in plasticised forms such as SEMTEX), hence will not explode unless properly detonated by an initiation charge. However it is sensitive enough that the initiation charge does not have to be very powerful.
- It has an extremely low 'vapourpressure', meaning at normal human temperature it produces very little vapour and cannot easily be detected by sniffers or dogs.
- Due to its widespread use as a military and commercial explosive, it can be legally obtained relatively easily.

In December last year, Umar Farouk Abdulmutallab flew from Lagos to Amsterdam and then boarded a plane bound for Detroit with PETN and another explosive apparently sewn into his underwear.

A controlled explosion carried out last year by UK explosives expert Sidney Alford found that just six grams of PETN punched a large circular dent into a metal plate twice the thickness of an aircraft fuselage.

Explosives 'would have detonated'

"We were lucky this was intelligence driven," Roland Alford, explosives expert and director of Alford Technologies told Channel 4 News.

"If [the bomb attack] hadn’t been uncovered, the explosives would have certainly have gone off."

Mr Alford said the high explosive was easy to get hold of and easy to set off - making it popular for terrorists making homemade bombs.

"PETN has the advantage to be easy to set off," he said. "If you've got access to explosives it is relatively easy to get.

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"PETN is one of the most common explosives for quarrying and all sorts of things. It comes in the form of detonation cord (det cord) which is used to connect between charges. If a detonator is then added and connected to a battery, it will detonate.

"Det cord contains PETN in powder form. If you cut that open it is very easy to get out pure PETN powder. There is about 10g of PETN per meter of det cord. Det cords can be 250m long so that’s 2.5kg of PETN in one reel.

"Looking at the size and density [of the explosives in the printer] I would estimate that there was close to about 500g of PETN, which is quite a lot of explosive."

500g of PETN explosives would destroy a room or a small building.

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Trigger mechanism

A dissembled mobile phone is thought to have been used in the device as a trigger mechanism.

Mr Alford said that as it was unlikely a mobile phone would work 30,000 feet in the air, the device could have been detonated via components in the mobile phone such as a timer.

As was the case in the Madrid train bombings in 2004, the devices could have been rigged to explode via an alarm on the mobile phone, rather than an incoming call.

Security concerns

"PETN is not difficult to detect," Mr Alford told Channel 4 News. "But the security problem came down to the way bomb had been disguised."

The explosives expert said a swab would have identified PETN straightaway - but with thousands of cargo packages passing through airports, the only guaranteed security check would be to individually check and s each parcel by hand.

"New technologies are being tested and introduced for X-rays where the density of explosive material can be identified," Mr Alford said.

He added that the disguise was particularly sophisticated. "The bombs are made to look like something else," he said. "I would challenge anyone to put a regular printer and one with this type of bomb in it through a scanner, and say which one has the device in it.

"It was very well concealed."