Adrian Peterson Investigated for Sexual Assault ... and Cleared

NFL star Adrian Peterson was the target of a criminal sexual assault investigation, but authorities ultimately cleared him ... TMZ Sports has learned.

Law enforcement sources tell us ... the alleged incident occurred in 2011 at a party in Eden Prairie, a suburb of Minneapolis. We're told ... the woman complained to authorities and Adrian denied anything occurred without the woman's consent.

A criminal investigation was opened, and numerous witnesses were interviewed. Peterson's lawyer, Peter Wold, tells TMZ ... "all witnesses who we interviewed concurred with Adrian's story." Further, Wold tells us ... authorities tested 3 DNA samples ... 2 excluded Peterson and one was inconclusive.

Wold also says Peterson submitted to a polygraph test and the results showed he was truthful and the authorities closed the case and no charges were ever filed.

The attorney calls the allegation "a bogus story that has nothing to do with reality."