Sanford was placed on the team's injured reserve list with a strained quadriceps muscle prior to the start of the season.
According to the Pioneer Press, Sanford said on Aug. 30 he was "frustrated" by the IR designation. He was unavailable for comment following Monday's release. The newspaper says Vikings coach Mike Zimmer didn't clearly answer a question last week about Sanford's status with the team.
The release of Sanford comes after the Vikings asked the sixth-year safety to take a pay cut during the offseason. ESPN, which described Sanford's relationship with the Vikings as "tenuous," says the veteran saw a $750,000 reduction in his salary in exchange for $400,000 of guaranteed money in his $1.75 million base salary.
The website says Sanford was unable to participate in the Vikings' organized team activities or minicamp, and suffered from a string of injuries during training camp and the preseason.
The Star Tribune says Sanford – a seventh-round draft pick from Mississippi in 2009 – started 44 of the 70 games he played for the Vikings. He played in 13 or more games in each of the last three seasons.
The Pioneer Press says the release will give Sanford the opportunity to join another team this season.