Some suggested the e-mails were used deliberately to skirt scrutiny of official White House e-mail.
"We're learning that off-book communications are being used by these people in the White House by using Republican political e-mail addresses, and they say they have not been preserved," Sen. Patrick Leahy, D-Vt., chairman of the Senate Judiciary Committee, said on the Senate floor. "I don't believe that! You can't erase emails -- not today."
Leahy compared the missing e-mails to the notorious 18-minute gap in President Nixon's White House tapes, discovered during the Watergate investigation.
The political maelstrom caused by the lost e-mails is merely one of the latest in a series of damaging disclosures related to the firing of the U.S. attorneys.
The revelation comes as two investigative congressional committees have asked the White House to turn over such e-mails.
Democrats on Capitol Hill have questioned whether the firings were politically motivated and whether Attorney General Alberto Gonzales, White House political adviser Karl Rove and White House counsel Harriet Miers were more closely involved than the administration has acknowledged.
A document released Friday could cause some difficulty for Gonzales' former chief of staff, Kyle Sampson, who told members of Congress that "with the exception of Bud Cummins, none of the U.S. attorneys was asked to resign in favor of a particular individual who had already been identified to take the vacant spot."
The e-mail message by Sampson, dated May 11, 2006, lists Rachel Brand as a candidate for a U.S. attorney's seat in the western division of Michigan and Tim Griffin for a post in the Eastern Division of Arkansas.
"There was some early brainstorming about possible replacements for one or more of the U.S. attorneys who were ultimately terminated. By the time they were let go in December 2006, no specific replacements had been identified or decided upon among any of the seven," one source with knowledge of the discussions about the firings told ABC News.