They're concerned with the casual nature of the comment though, and the fact that it came after the communication/tracking controls or whatever it is was turned off. The pilot wad supposed to sign off properly, citing the plane's call sign and instructions for contacting the next tower. So they're going to analyse his voice for stress to see if they had been taken over prior to this and maybe he did it 'wrong' to alert them that something wasn't right.

That's what I took from the article anyway.