Using intermediaries and inexpensive computer discs, Osama bin Laden managed to send emails while in hiding, without leaving a digital fingerprint for United States eavesdroppers to find.

His system was painstaking and slow, but it worked, and it allowed him to become a prolific email writer despite not having Internet or phone lines running to his compound.

His methods, described in new detail to The Associated Press by a counter-terrorism official and a second person briefed on the US investigation, frustrated Western efforts to trace him through cyberspace. The people spoke to the AP on condition of anonymity to discuss the sensitive intelligence analysis.

Bin Laden's system was built on discipline and trust.

But it also left behind an extensive archive of email exchanges for the US to scour. The trove of electronic records pulled out of his compound after he was killed last week is revealing thousands of messages and potentially hundreds of email addresses, the AP has learned.

At that location, the courier would plug the memory drive into a computer, copy bin Laden's message into an email and send it. Reversing the process, the courier would copy any incoming email to the flash drive and return to the compound, where bin Laden would read his messages offline.

It was a slow, toilsome process. And it was so meticulous that even veteran intelligence officials have marveled at bin Laden's ability to maintain it for so long.

The US always suspected bin Laden was communicating through couriers but did not anticipate the breadth of his communications as revealed by the materials he left behind.

Navy SEALs hauled away roughly 100 flash memory drives after they killed bin Laden, and officials said they appear to archive the back-and-forth communication between bin Laden and his associates around the world.