Assange and his mother both hinted at it. One interview Assange did is particularly notable; he practically said the leaker was Seth Rich and then backed off a bit. It was like he was struggling with his promise to not reveal sources.

Seth Rich had the means to leak, being an IT person just like Snowden was. Seth Rich had the motive to leak, being a very upset Bernie supporter. In one of John Podesta's emails, there were some very ominous words about making an example of leakers.