In a high security mental hospital, but while he was being transported. The prison bus crashed, killing the driver, and rendering the 4 officers unconscious.
The prisoner escaped and hasn't been seen since. His murders are still being committed, in the exact same way. No one knows were he is hiding, or if it is him committing the murders.
Some believe he died in the accident, and its a copy cat committing the murders. Others believe he is still alive, and is still killing people.