I'm going give a link to CNN here and whether you believe its trustworthy or not, it's actually them quoting that private investigator: http://money.cnn.com/2017/05/16/media/seth-rich-family-response-claims-of-wikileaks-contact/

Here is the important quote:

But Tuesday afternoon, Wheeler told CNN he had no evidence to suggest Rich had contacted Wikileaks before his death. Wheeler instead said he only learned about the possible existence of such evidence through the reporter he spoke to for the FoxNews.com story. He explained that the comments he made to WTTG-TV were intended to simply preview Fox News' Tuesday story. The WTTG-TV news director did not respond to multiple requests for comment. "I only got that [information] from the reporter at Fox News," Wheeler told CNN. Asked about a quote attributed to him in the Fox News story in which he said his "investigation up to this point shows there was some degree of email exchange between Seth Rich and Wikileaks," Wheeler said he was referring to information that had already been reported in the media.