Meanwhile, "Evidence Eliminator" software installed by Mr Beale deleted documents which were likely to have been relevant to the investigation.

That count centers on Mr. Quattrone's having sent an e-mail message to employees telling them to delete documents.

Defragmenting your hard drive and deleting old files and documents to free disk space might help.

For example, storage management tasks assist when creating, reading, editing, moving, copying and deleting documents.

The same concern applies if employees suddenly start shredding and deleting documents, even in compliance with company policy, that they had previously let pile up, he said.

The "limbo file" facility enabled users to recover accidentally deleted documents until the disk ran out of space, when the software would permanently delete files to make room for new ones.

Every action taken by a computer user leaves a telltale trail, he said, so the act of deleting documents can itself be revealing.

They combine data storage space with software that goes beyond document imaging to include functions such as audit trail and reporting tools, document expiration controls to delete documents at specified dates and Web-based document viewing.

While the contract makes it clear that Mr. Giuliani cannot legally shred or delete documents without city approval, his own history does not breed confidence that his hand-picked archivists would err on the side of openness.

Critics asserted that the e-mails showed that scientists were conspiring to delete e-mails and documents to prevent them from being released.