Despite having no internet access in his hideout, Osama bin Laden was a prolific email writer who built a painstaking system that kept him one step ahead of the U.S. government's best eavesdroppers.

His methods, described in new detail to The Associated Press by a counterterrorism official and a second person briefed on the U.S. investigation, served him well for years and frustrated Western efforts to trace him through cyberspace. The arrangement allowed bin Laden to stay in touch worldwide without leaving any digital fingerprints behind.

The people spoke to the AP on condition of anonymity to discuss the sensitive intelligence analysis.

Osama bin Laden went to great lengths to send messages over the internet to supporters. (Associated Press)

Bin Laden's system was built on discipline and trust. But it also left behind an extensive archive of email exchanges for the U.S. 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.

Holed up in his walled compound in northeast Pakistan with no phone or internet capabilities, bin Laden would type a message on his computer without an internet connection, then save it using a thumb-sized flash drive. He then passed the flash drive to a trusted courier, who would head for a distant internet cafe.

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 U.S. 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.

Al-Qaeda operatives are known to change email addresses, so it's unclear how many are still active since bin Laden's death. But the long list of electronic addresses and phone numbers in the emails is expected to touch off a flurry of national security letters and subpoenas to internet service providers.

The Justice Department is already coming off a year in which it significantly increased the number of national security letters, which allow the FBI to quickly demand information from companies and others without asking a judge to formally issue a subpoena.

Officials gave no indication that bin Laden was communicating with anyone inside the U.S., but terrorists have historically used U.S.-based internet providers or free internet-based email services.

The cache of electronic documents is so enormous that the government has enlisted Arabic speakers from around the intelligence community to pore over it. Officials have said the records revealed no new terror plot but showed bin Laden remained involved in al-Qaeda's operations long after the U.S. had assumed he had passed control to his deputy, Ayman al-Zawahri.

The files seized from bin Laden's compound not only have the potential to help the U.S. find other al-Qaeda figures, they may also force terrorists to change their routines. That could make them more vulnerable to making mistakes and being discovered.