The manifesto, a PDF file containing the shooter's name, was posted to 8chan before first reports of the massacre — and before his name was publicly known.
"I'm probably going to die today," says the posting to which it was attached. The shooter surrendered to police after the killings.
Facebook, which owns Instagram, said the company found nothing that supported Watkins' claim. The company also said it disabled the Instagram account associated with the shooter on Saturday, adding that the account had not been active in more than a year.
8chan remained unreachable on the web on Wednesday. Its administrator tweeted Tuesday that he was moving the site onto the darknet, an online corner of the internet hidden from normal browsing where illicit drugs are sold and criminals trade stolen credit card data. That would make it significantly more cumbersome to access — and unappealing for mass use.
The site was knocked offline after Cloudflare said it was cutting it off from protection from denial-of-service attacks, for being "a cesspool of hate."
8chan quickly found new online backers in Washington state-based Epik.com and its BitMitigate security service. But on Monday, the company that provided them with computer servers and other technical underpinnings, London-based Voxility, upended both Epik and BitMitigate by terminating its contract with them.