'This is a case where we have a trust with the service members who have served us and ... we need do justice,' Ash Carter told NBC News.
Two Fort Campbell soldiers are being held as persons of interest in the disappearance of Shadow McClain.
A senior police official said the siblings were both aged in their 30s and originally from Michigan.
Two children escaped from the Australian theme park accident in which 4 adults died. Robert Ovadia of 7 News Australia reports.
Raqqa is the de facto capital of the extremists' so-called caliphate which stretches from Syria into Iraq.