But Pennsylvania Attorney General Kathleen Kane said the investigation should not have taken three years. She said prosecutors made "crucial missteps" and only picked up the pace in the final year.
"The facts show an inexcusable lack of urgency in charging and stopping a serial child predator," Kane said in a statement.
Sandusky, the longtime assistant to Joe Paterno, Penn State's admired head coach, is currently serving 30 to 60 years for molesting boys he met through his Second Mile charity. Paterno, who was dismissed in 2011, died a few months later.
While Sandusky retired in 1999, he was given an emeritus title and still had an office at the university. There had been questions about his conduct for years and an investigation in 1998 that ended without charges being filed, but the investigation that led to his eventual arrest only began in 2009.
Tom Corbett, who was elected governor in 2010, was attorney general at the time. The report found "no direct evidence that electoral politics influenced any important decision made in the Sandusky investigation."
Geoffrey Moulton, a law professor who formerly served as a federal prosecutor, spent 16 months preparing the report. He questioned why the Pennsylvania Attorney General's Office waited until December 2010 to subpoena records from Penn State and until February 2011 to search Sandusky's house.
Kane, a Democrat, was elected attorney general in 2012. She criticized Corbett's handling of the investigation during the campaign, suggesting the Republican governor put political considerations first.