Ohio officials say criminal and internal investigations are underway to determine how three inmates, including a convicted killer of three students, escaped from prison.

State correction officials say outside experts also will examine prison procedures and recommend possible improvements.

The state says the inmates, including school shooter T.J. Lane, escaped from prison in Lima (LY'-muh) on Thursday night by scaling a fence to a roof, which immediately sounded a fence alarm.

All three were back in custody by Friday morning, and officials say they have been transferred to a high-security prison in Youngstown.

The union representing prison guards says officials learned of an escape plan Wednesday and segregated an inmate from the same unit as the three escapees. A prisons spokeswoman says that inmate had nothing to do with Thursday's escape.