A tweet from the account of the far-right activist who organized the Charlottesville, Va., "Unite the Right" rally insulted the protester who was killed at the event, saying late Friday night that her death was "payback time."
The tweet linked to a story on neo-Nazi website the Daily Stormer that also insulted Heyer in crude terms and appeared to take glee in her death.
Kessler did not respond to messages seeking comment.
Police say Heyer was killed when a rally attendee, James A. Fields, 20, drove his sports car into a crowd of counter-protesters at the event Aug. 12, which drew white nationalists, neo-Nazis and other far-right figures from around the nation.
Fields has been charged with her murder. Kessler had blamed city officials for not providing sufficient security for the rally, which originally was organized to protest the removal of a statue of Confederate Gen. Robert E. Lee from a Charlottesville park.
Kessler's tweet sparked denunciations from other far-right rally attendees, who quickly distanced themselves, accelerating a spiral of recriminations that have been brewing among far-right leaders over who was to blame for the chaos behind last weekend's violent "Unite the Right" rally.
On Saturday morning, the tweet had been deleted from Kessler's account, which initially claimed he'd been hacked but then backtracked and said he'd been on a mixture of drugs.
"I repudiate the heinous tweet that was sent from my account last night. I've been under a crushing amount of stress & death threats," the tweet stated. "I'm taking ambien, xanax and I had been drinking last night. I sometimes wake up having done strange things I can't remember."
Kessler's tweets then were switched to "private" mode before his account was deleted entirely.
"I will no longer associate w/ Jason Kessler; no one should," tweeted Richard Spencer, a white nationalist who was scheduled to speak at Kessler's event. "Heyer's death was deeply saddening. 'Payback' is a morally reprehensible idea."
Another far-right figure who attended the event, Tim Gionet, who goes by the name Baked Alaska, also criticized the remarks.
"This is terribly wrong and vile," Gionet tweeted. "We should not rejoice at the people who died in Charlottesville just because we disagree with them."
"Assuming this is a real tweet and his account was not hacked, I will no longer attend or cover events put on by Jason Kessler," tweeted rally attendee James Allsup. "Very gross."
"Why. Would You. Tweet This," another popular far-right account, @FaustianNation, tweeted at Kessler. "This tweet makes it impossible to defend you, and now the entire rally as you were the main organizer."
Adding to the confusion, a user purporting to be Daily Stormer staffer Andrew "Weev" Auernheimer claimed on the social media service Gab that he hacked Kessler's account, though his claim could not be confirmed.
Auernheimer had his own ax to grind: He posted his suspicions that other white nationalist organizers, including Spencer, had sought to block a neo-Nazi contingent with the Daily Stormer from attending the event.
White nationalists and others who attended the event have been subject to an intense public crackdown after last weekend's rally ended with brawls and Heyer's death.
President Trump also drew criticism when he suggested in a news conference that there were "very fine people" among the far-right figures who were protesting the removal of the Confederate statue.
Matt Pearce is a national reporter for The Times. Follow him on Twitter at @mattdpearce.
11:55 a.m.: This article was updated with a tweet from Kessler's account stating he had been drinking and taking drugs and news that the account had been deleted.
Aug. 19, 5 a.m.: This article was updated with Kessler tweeting that he'd been hacked.