According to TMZ, Kelly died in her sleep at a California rehab facility.
Her agent told the website, "She had been fighting demons for a while and finally lost her battle."
The actress struggled with substance abuse and voluntary checked herself into rehab, hopping to kick her habit.
Kelly's boyfriend reportedly brought her to the rehab facility on Monday, but she went into cardiac arrest Wednesday night and was not able to be saved.
Sources say paramedics found needle marks on both of Kelly's arms when they arrived on scene.
But according to her boyfriend, the marks were from a previous hospital visit where nurses made multiple attempts to extract blood from her arm.
More information will follow as this story develops.