Junior safety Janzen Jackson, one of Tennessee's most talented and experienced defenders, has been dismissed from the team.

The Vols dismissed junior safety Janzen Jackson, a second-team All-SEC selection, from the team. Joe Robbins/Getty Images

Coach Derek Dooley announced Jackson's dismissal Wednesday. Dooley declined to divulge specifics, but sources told ESPN.com that Jackson has battled substance-abuse issues during his time at Tennessee and failed multiple drug tests.

"Our program has devoted a tremendous amount of energy, resources, support and care in an effort to help Janzen manage his personal challenges," the coach said in a statement. "I will always be there to help him as a person, but there comes a time when a player's actions preclude him from the privilege of playing for the University of Tennessee football team.

"Although I'm disappointed with this outcome, we will never compromise the long-term organizational values and goals we maintain here at Tennessee."

Jackson, a second-team All-SEC selection, withdrew from school in February to deal with what Tennessee officials said were personal issues. He also missed the pre-Christmas practices on campus prior to the Franklin American Mortgage Music City Bowl to tend to personal matters, but played in the game.

Dooley had gone to great lengths to help Jackson and vowed to leave the door open as long as Jackson continued to try and take care of his issues. All the while, Dooley kept saying even after Jackson was reinstated in July, just prior to preseason practice, that it was day to day with the player and that he was under a zero-tolerance policy.

Jackson tied for the team lead last season with five interceptions and led all Vols defensive backs with 69 tackles.

He had made 22

starts in 23 games in two seasons, and was expected to be a key part of the secondary.

Chris Low covers the Southeastern Conference for ESPN.com.