Mauro Chialli - one of eight jurors who acquitted Knox and her ex-boyfriend Raffaele Sollecito of charges they sexually assaulted and murdered Meredith Kercher in 2007 - said he had watched their faces closely during the appeal, Fox News reports.
Knox returned home to Seattle on Tuesday, and Sollecito to his home in southern Italy.
"I saw the faces of these two kids, and they couldn't bluff. They didn't bluff. My point of view is that these kids weren't guilty. They weren't there," Chialli said on Friday during an interview on Italy's state-run RAI television.
Knox and Sollecito were convicted in 2009 and sentenced to 26 and 25 years respectively.
A third defendant, Rudy Hermann Guede, lost his appeal and is serving 16 years in jail. (Read more at Globalpost.com)
Chialli said the lack of a motive and uncertainty around when Kercher died meant he was not convinced of the pair's guilt.
"What didn't convice me was that in the end, it was an accusation based on so many conjectures," he said. "It could have been this way, it could have been another way," he said, the UK's Daily Mail reports.
Knox and Sollecito told the jury they were at Sollecito's apartment the night of the killing, smoking hashish, watching a film and having sex.