A new report claims that zero-emissions vehicle startup Nikola Motor has made a series of deceptive public statements and representations about its technology and business. Allegations include that Nikola staged a 2018 video of its signature hydrogen fuel-cell truck driving, and that it has made deceptive claims about its battery development efforts after the failure of an acquisition deal.

The allegations come from Hindenburg Research, which has a short position in Nikola stock. But they follow previous reports of misrepresentations by Nikola and broader expert skepticism about the company’s aggressive claims.

The Hindenburg report’s most striking claim is that a January 2018 video purportedly showing a Nikola One hydrogen fuel-cell semi truck moving under its own power was staged. According to Hindenburg, the video in fact showed the truck rolling down a long, gentle slope. Hindenburg’s report includes a test confirming that the section of road shown in the video could accelerate a coasting vehicle to highway speeds, along with text messages from a former Nikola employee appearing to confirm the tactics.

Given these claims, it is notable that Nikola repeatedly described the video as showing the truck “in motion,” which would be technically true even if the truck were not moving under its own power. Bloomberg had previously reported that Nikola and its founder, Trevor Milton, made deceptive claims about the capabilities of the Nikola One unit displayed at a 2016 event.

Another major claim made in the Hindenburg report is that Nikola and Milton have misrepresented the company’s efforts to develop a new kind of battery for use in electric vehicles.

In November 2019, Milton claimed that Nikola would soon unveil “the biggest advancement we have seen in the battery world.” Those claims appear to have been based on the planned acquisition of a company called ZapGo, but by December, Nikola had backed out of that acquisition. In a March 2020 legal filing, Nikola said ZapGo “had nothing more than interesting research with no ability to commercialize its research.”

Despite pulling out of the deal, Nikola did not revise its public messaging about an impending revolution in EV battery technology. In a June interview with Fortune and in other public statements, Milton continued to promise an imminent unveiling of the tech. Milton claimed that Nikola was working to commercialize battery technology it had licensed from academic researchers, while declining to name those researchers. Experts evaluating Nikola’s battery claims were skeptical of the company’s aggressive timeline.

Further questions about Nikola’s prior battery claims were raised by a September 8 announcement that Nikola would use battery technology licensed from General Motors in vehicles including its upcoming Badger pickup truck. That announcement caused both Nikola and GM stock to rise dramatically. On September 10, the Hindenburg report appears to be driving both stocks back down, with Nikola off nearly 10% at press time, and GM down nearly 4%.

The Hindenburg report also makes a variety of other claims about deception by Nikola and Trevor Milton, including alleged misrepresentations by Milton about prior companies he founded and sold.

Nikola sent Fortune a brief statement in response to Hindenburg's report: “Nikola has been vetted by some of the world’s most credible companies and investors. We are on a path to success and will not waver based on a report filled with misleading information attempting to manipulate our stock.”

Nikola did not address Fortune’s questions about Hindenburg’s specific claims, including questions about whether the truck shown in the 2018 video was moving under its own power, or who Nikola's battery development partners were by summer 2020. On Twitter, Milton wrote, "It will take the rest of the day to address the one sided false claims, but I will put out a detail report to address it. In the meantime, troll on."

Though its short position would allow it to benefit from negative stock moves, Hindenburg Research has built a strong reputation in recent years based on in-depth research into public misrepresentations and accounting irregularities at companies including Smile Direct Club, SC Worx, and Bloom Energy. Many of Hindenburg's claims about those and other companies were subsequently confirmed by other reporting, by regulators, or by the companies themselves.