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Investors are pulling the plug on Bloom Energy (BE) after a scathing report by a short seller raised questions about the energy company's technology and accounting practices.

Shares of Bloom Energy plunged 21% to $3.31 after Hindenburg Research claimed to have found $2.2 billion in undisclosed liabilities.

Bloom Energy has used "tricky accounting" to "mask servicing costs and shift write-downs to other periods, thereby avoiding recognizing major recent additional losses," Hindenburg contends.

Hindenburg also took aim at Bloom Energy's much-touted electricity-generating fuel-cell technology, saying the Bloom Boxes are no cleaner that a natural-gas-fired power plant.

Founded in 2001 by a former NASA scientist, Bloom Energy "has been unable to fulfill its ambitions of being profitable," the Hindenburg report notes, while blasting as "absurd" the company's "clean energy narrative."

"We expect Bloom Energy will become yet another tombstone in the Silicon Valley cemetery of dead unicorns," the short seller argues in its report.

Pushing back, Bloom Energy said in a statement that Hindenburg did not "engage with Bloom Energy to validate the assertions made in their report."

The company also blasted Hindenburg's claims about "service replacement liability," calling them "grossly misleading."

"We strongly disagree with the conclusions drawn," the company said in a statement, adding it will have more to say later.