Army Pfc. Bradley Manning is escorted from a security vehicle to a courthouse in Fort Meade, Maryland, on Monday, Dec. 19, 2011. Photo: Patrick Semansky/AP

FT. MEADE, Maryland — A day after a government forensic expert testified that he’d found thousands of diplomatic cables on the Army computer of suspected WikiLeaks source Bradley Manning, he was forced to admit under cross-examination that none of the cables he compared to the ones WikiLeaks released matched.

Special Agent David Shaver, a forensic investigator with the Army’s Computer Crimes Investigations Unit, testified Sunday that he’d fou...