from the background noise of about 200,000 other mobile telephones within a kilometre of police headquarters.
They used a technique called Random Frequency Tracking System. The technique was not uncommon. It had been developed by the U.S. National Security Agency, and was built into an unknown number of satellites that performed pinpoint monitoring of capitals around the world as well as flashpoints of special interest.
The N.S.A. had enormous resources and used a vast network in order to capture a large number of mobile conversations in a certain region simultaneously. Each individual call was separated and processed digitally by computers programmed to react to certain words, such as terrorist or Kalashnikov. If such a word occurred, the computer automatically sent an alarm, which meant that some operator would go in manually and listen to the conversation to decide whether it was of interest or not.
It was a more complex problem to identify a specific mobile telephone. Each mobile has its own unique signature - a fingerprint - in the form of the telephone number. With exceptionally sensitive equipment the N.S.A. could focus on a specific area to separate out and monitor mobile calls. The technique was simple but not 100 per cent effective. Outgoing calls were particularly hard to identify. Incoming calls were simpler because they were preceded by the fingerprint that would enable the telephone in question to receive the signal.
The difference between Trinity and the N.S.A. attempting to eavesdrop could be measured in economic terms. The N.S.A. had an annual budget of several billion U.S. dollars, close to twelve thousand fulltime agents, and access to cutting-edge technology in I.T. and telecommunications. Trinity had a van with thirty kilos of electronic equipment, much of which was home-made stuff that Bob the Dog had set up. Through its global satellite monitoring the N.S.A. could home in highly sensitive antennae on a specific building anywhere in the world. Trinity had an antenna constructed by Bob the Dog which had an effective range of about five hundred metres.
The relatively limited technology to which Trinity had access meant that he had to park his van on Bergsgatan or one of the nearby streets and laboriously calibrate the equipment until he had identified the fingerprint that represented Ekstrom's mobile number. Since he did not know Swedish, he had to relay the conversations via another mobile back home to Plague, who did the actual eavesdropping.
For five days Plague, who was looking more and more hollow-eyed, listened in vain to a vast number of calls to and from police headquarters and the surrounding buildings. He had heard fragments of ongoing investigations, uncovered planned lovers' trysts, and taped hours and hours of conversations of no interest whatsoever. Late on the evening of the fifth day, Trinity sent a signal which a digital display instantly identified as Ekstrom's mobile number. Plague locked the parabolic antenna on to the exact frequency.
The technology of R.F.T.S. worked primarily on incoming calls to Ekstrom. Trinity's parabolic antenna captured the search for Ekstrom's mobile number as it was sent through the ether.
Because Trinity could record the calls from Ekstrom, he also got voiceprints that Plague could process.
Plague ran Ekstrom's digitized voice through a program called V.P.R.S., Voiceprint Recognition System. He specified a dozen commonly occurring words, such as "O.K." or "Salander". When he had five separate examples of a word, he charted it with respect to the time it took to speak the word, what tone of voice and frequency range it had, whether the end of the word went up or down, and a dozen other markers. The result was a graph. In this way Plague could also monitor outgoing calls from Ekstrom. His parabolic antenna would be permanently listening out for a call containing Ekstrom's characteristic graph curve for one of a dozen commonly occurring words. The technology was not perfect, but roughly half of all the calls that Ekstrom made on his mobile from anywhere near police headquarters were monitored and recorded.
The system had an obvious weakness. As soon as Ekstrom left police headquarters, it was no longer possible to monitor his mobile, unless Trinity knew where he was and could park his van in the immediate vicinity.
With the authorization from the highest level, Edklinth had been able to set up a legitimate operations department. He picked four colleagues, purposely selecting younger talent who had experience on the regular police force and were only recently recruited to S.I.S. Two had a background in the Fraud Division, one had