Published online 16 January 2004 | Nature | doi:10.1038/news040112-15


Bullets found in airport arrest

Can technology stop passengers carrying weapons on board?

Metal bullets should be easily spotted by x-rays.Metal bullets should be easily spotted by x-rays.©

Just as the United States has brought in new security measures for international flights - including fingerprint screening for some passengers - a scare has shown that even the most basic existing security checks can fail.

Security officials at Washington DC's Dulles International Airport this week allegedly allowed a passenger to carry five bullets aboard a flight to London. The ammunition was discovered in the suspect's pocket when his coat was put through the hand-baggage X-ray machine at London Heathrow airport, as he attempted to board a connecting flight.

"We're stunned that this passenger got through security at Washington," says Carolyn Evans, head of flight safety for the British Airline Pilots' Association. No one yet knows how that happened.

X-ray machines in most airports are designed such that different materials, such as metals and organic substances, show up in different colours on the screen, helping staff to pick out suspicious objects. Although it can be difficult to unpick such a picture when it contains a jumble of objects, a handful of dense metal objects such as bullets in a pocket should be one of the easiest things to spot, says Andy Lamnea, a technical expert with L-3 Communications, a New York-based company that supplies surveillance equipment.

But it is obviously impossible to completely eliminate human error - especially as security staff can be easily lulled into boredom by the endless stream of bags and coats that scroll across the X-ray screen, says John Head, an independent security consultant in Burgess Hill, UK.

To help keep security officials on their toes, Qinetiq, a technology company based in Farnborough, UK, developed a computer program that can superimpose fake images of objects such as guns onto real X-ray scans of bags. The system, called Threat Image Projection, both keeps security workers awake and provides a way to check up on their accuracy. It is already used at 14 of Britain's biggest airports, and similar systems are used in other airports around the world.

X-ray technology is also improving, says Lamnea. Future machines could be designed to highlight explosives, for example.

And, of course, there are a host of other technologies - both existing and in development - that can be used to detect contraband.

Metal detectors

Not all metal detectors are equal.Not all metal detectors are equal.© Corbis

If the man had carried bullets in his trouser pocket, the metal-detecting archways should have picked them up. The archways are designed to trigger when any amount of any kind of metal passes through. The metal develops a magnetic field in the presence of short bursts of electric current travelling through a wire coil on one side of the arch. This should interfere with the larger magnetic field set up by the arch itself, creating a detectable blip in the reflected pulse of current.

But not all archways are equal. Whereas all are set to comply with a defined lower limit of sensitivity, airports can chose to turn the volume up. At the other end of the scale, anecdotal evidence suggests some passengers have passed through with a few coins in their pockets without setting off alarms.

Millimetre-wave cameras

One option for detecting weapons in the near future could be cameras that see through clothing. Millimetre-wave cameras detect radiation at longer wavelengths than both visible light and the infrared radiation used by 'night vision' goggles. At wavelengths of a few millimetres, clothes are invisible but dense items such as bullets, ceramic knives and human bodies are clearly shown. Unlike an X-ray, the device doesn't subject passengers to a blast of radiation, but detects naturally existing waves. Such cameras are already used to detect stowaways in trucks travelling through the Eurotunnel between England and France, for example, and could find their way into airports too.


If the problem is detecting explosives rather than bullets, the solution could be a new variety of electronic nose. Heathrow Airport has recently been testing a system where selected passengers walk into a cubicle slightly wider than a phone box and wait for about 10 seconds while strong jets of air blow up from the floor. The air carries traces of chemicals on the person's clothing into a chimney at the top of the machine, where they are tested for evidence of explosives residues. Smaller 'sniffer' devices are already used in many airports to detect trace chemicals on luggage.


An iris is one of the most accurate biometric measures.An iris is one of the most accurate biometric measures.© Corbis

Biometrics could provide a way to catch suspect passengers before they make it to the security gates. Since 2001, Amsterdam's Schiphol Airport in the Netherlands has been routinely using an iris recognition system on certain passengers. The scanning unit takes less than a second to take four digital photographs of the eye, which are compared with data stored on a smart card to confirm the identity of the passenger. Schiphol claim that the iris is more unique than a fingerprint, making it one of the most accurate biometric measures. But the system can only determine the identity of people already on record, and can't tell the intentions of the person it checks.

The human factor

In the end, human error is the most likely reason for these systems to fail. But taking humans out of the loop is not an option. People are often the most sensitive detection devices available, easily able to spot someone who is acting suspiciously. And with all the technological barricades between a passenger or bag and an airplane, you still need someone to physically rummage through the bags.