Not since the glory days of letter-writing, before the advent of the telephone, have people committed so much revealing stuff to written form as they do in the age of computers.

All those e-mail messages and electronic files are a treasure trove of evidence for law enforcement officers, whether they are targeting terrorists, crooked CEOs or local drug dealers.

The challenge for police and prosecutors is learning how to dig up and preserve these electronic gems.

"Any agent can come in and look through papers, but not every agent can do a thorough computer search," said David Green, deputy chief of the Justice Department's computer crime section, which helps train federal and state investigators.

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