(Reuters) – Short seller Hindenburg Research on Thursday initiated a short position in Icahn Enterprises LP bonds and said the investment firm led by billionaire Carl Icahn has failed to address every key issue it had raised.

Hindenburg’s latest move comes a day after the company reported a surprise loss and disclosed it was contacted by U.S. prosecutors, though it was not clear if it was related to the its allegations made last week.

It had criticized the investment firm of over-reporting its finances and accused it for relying on a “Ponzi-like” structure to pay dividends. Icahn called the report “self-serving” and vowed to take steps to “fight back”.

Hindenburg’s reports are a setback for Icahn, a pioneer of shareholder activism known for taking on heavyweights such AIG (NYSE:AIG) and McDonald’s Corp (NYSE:MCD) and raising issues related to governance and transparency.

Icahn’s latest disclosures raise critical new questions about margin loans and continued portfolio losses, the short seller said on Thursday, sending the investment firm’s shares down over 6%.

“We are short units of Icahn Enterprises and have initiated a short position in IEP bonds,” said the short seller whose reports on companies have often erased a big chunk of their value.

Failure to disclose basic details about the margin loans represent a “near-term critical threat to IEP unitholders,” Hindenburg alleged.

Icahn Enterprises did not immediately respond to a request for comment.

Icahn owns about 84% of IEP and had pledged more than 65% of his stake as collateral for margin loans for “unspecified purposes”, the short seller said.