Cómo se caza a terroristas

A veces nos preguntamos cómo pueden los servicios de inteligencia o las fuerzas de seguridad llegar hasta altos líderes terroristas. En esta historia que aparece hoy en el New York Times se ve un ejemplo:

The terrorism investigation code-named Mont Blanc began almost by accident in April 2002, when authorities intercepted a cellphone call that lasted less than a minute and involved not a single word of conversation.
Investigators, suspicious that the call was a signal between terrorists, followed the trail first to one terror suspect, then to others, and eventually to terror cells on three continents.

What tied them together was a computer chip smaller than a fingernail.
[...]
For two years, investigators now say, they were able to track the conversations and movements of several Qaeda leaders and dozens of operatives after determining that the suspects favored a particular brand of cellphone chip. The chips carry prepaid minutes and allow phone use around the world.