A group of cybersecurity experts has unearthed ties between an American hacker who maintains a neo-Nazi website and an internet campaign to smear Emmanuel Macron days before he was elected president of France.

Shortly after an anonymous user of the 4chan.org discussion forum posted fake documents purporting to show Mr. Macron had set up an undisclosed shell company in the Caribbean, the user directed people to visit nouveaumartel.com for updates on the French election.

That website, according to research by web-security provider Virtualroad.org, is registered by “Weevlos,” a known online alias of Andrew Auernheimer, an American hacker who gained notoriety three years ago when a U.S. appeals court vacated his conviction for computer fraud. The site also is hosted by a server in Latvia that hosts the Daily Stormer, a neo-Nazi news site that identifies its administrator as “Weev,” another online alias of Mr. Aeurnheimer, Virtualroad.org says.

“We strongly believe that the fake offshore documents were created by someone with control of the Daily Stormer server,” said Tord Lundström, a computer forensics investigator at Virtualroad.org.

Through Tor Ekeland, the lawyer who represented him in the computer-fraud case in the U.S., Mr. Auernheimer said he “doesn’t have anything to say.”