The founder and executive chairman of embattled zero-emission truck maker Nikola has resigned, the company announced on Sunday, after allegations of fraud which sparked a stock tumble and a regulatory investigation.

The Phoenix-based company said in a statement that it had accepted Trevor Milton’s resignation and that he would be replaced by Stephen Girsky, a Nikola board member and former vice-chairman at General Motors (GM).

Founded by Milton in 2015 to develop trucks and pick-ups powered by electric batteries or hydrogen fuel cells, Nikola has not yet built anything, but caught attention by signing strategic partnerships with such renowned groups as GM and German engineering giant Bosch.

The announcement of the GM partnership on September 8 caused shares to leap 41% on the New York Stock Exchange.

But two days later, investment company Hindenburg Research published a report accusing the startup of “intricate fraud” based on multiple lies by Milton, who it said “misled partners into signing agreements by falsely claiming to have extensive proprietary technology”.

That announcement triggered a plummet in share value, with stock diving 36% in three days. It also sparked an investigation by the Securities and Exchange Commission, according to sources cited by Bloomberg.

In a statement announcing his resignation, Milton said: “The focus should be on the Company and its world-changing mission, not me. I intend to defend myself against false allegations leveled against me by outside detractors.” Nikola had rejected most of the claims in the Hindenburg report.