LONDON, England (CNN) -- Mobile phones are in the hands of millions of people around the world. And increasingly, it appears, in the hands of terrorists.
The bombers who targeted commuter trains in Madrid on March 11 used the built-in alarm clock in mobile phones to set off explosives.
In Jerusalem, it is believed a call to a cell phone in a rucksack set off a bomb at Hebrew University in 2002, killing seven.
One of the Bali bombs outside the Sari nightclub in October 2002 had a cell phone attached, as did a car bomb which killed 12 people at the Jakarta Marriott hotel last August.
David Claridge, of the Risk Advisory Group, said: "Mobile phones are relatively cheap, you can acquire them in relatively large numbers and you can build a whole stack of them at one time and place them and set them off at your leisure.
"It means that you can step away some considerable distance, the other side of the world, in order to initiate the explosive device."
Searches following attacks in Riyadh, Saudi Arabia, last May led to an FBI warning about the use of cell phones, saying the modifications needed to turn a phone into a trigger were "relatively minor."
But there are products on the market to stem the use of mobile phones in terrorist attacks.
One phone-sized device can block cell phone signals in a 20 foot radius. And larger devices can block signals reaching an entire building.
But in Britain, the United States and many other countries, it is illegal for civilians to use cell phone blockers. The right to make a call reigns supreme.
Michael Menage, of Global Gadget, said: "In the UK, as in most other nations, they are illegal, because you are interfering with somebody else's right to use a mobile telephone. Through these aerials, you are broadcasting a signal on a wavelength for which you do not have a license."
But governments are using similar technology.
It is widely reported a radio jammer inside the car of Pakistani President Pervez Musharraf stopped an attempt to assassinate him last December.
And it is expected more high profile people will use the technology in future.
Peter Yapp, of Control Risks Group, said: "I would be surprised if some of these high-profile people aren't using this technology, certainly for cars, and for small areas like that."
The U.S. Department of Defense has plans to jam larger areas.
Codenamed "Wolfpack," the technology would deny the enemy the use of all radio communications, including mobile phones, on the battlefield.
But to deny the terrorists the use of a mobile phone would deny everyone from using one in a certain area.
Not practical at train stations, concert halls, sporting events and most buildings.
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