The investigation into who snooped into confidential passport files of famous Americans has resulted in a fourth criminal conviction.

Twenty-seven-year-old William Celey of Washington pleaded guilty Friday to unauthorized computer access. He admitted he looked at passport applications of more than 75 celebrities, politicians and family members out of curiosity while working as a State Department contractor in 2004.

The applications contain a photograph and personal information and are protected under federal privacy laws.

Three former State Department workers have been sentenced to probation in the case. An investigation began in March 2008 after officials discovered unauthorized access of the files for then-presidential candidates Barack Obama, John McCain and Hillary Rodham Clinton.