The boutique Los Angeles-based investment bank said last week that it was forced to mark down the value of its investment in the $2.6 billion take-private deal involving Vitamin Shoppe owner Franchise Group Inc after the company reported a decline in revenue and a net loss in the third quarter.

That markdown resulted in B. Riley reporting a net loss of $75.8 million in the third quarter, compared with a profit of $45.8 million a year earlier.

B. Riley's stock plunged to as low as $16.65 on Monday, shedding nearly 50% of its value since the announcement. Its shares have pared some of those losses and were last down nearly 14% to $21.90.

S&P Global Ratings downgraded Franchise Group's credit rating on Friday, dropping it further down in junk status to 'B-' from 'B' with a negative outlook, citing its weak performance. The rating agency also said it was monitoring legal developments relating to Franchise Group's Chief Executive Brian Kahn, who led the August take-private deal.

A B.Riley spokesperson did not immediately respond to a Reuters request for comment.

Bloomberg reported this month that Kahn is one of two co-conspirators named by co-founder of hedge fund Prophecy Asset Management John Hughes, who this month pleaded guilty to securities fraud. It's unclear if Kahn is also facing charges, Bloomberg reported.

"At no time during my former business relationship with Prophecy did I know that Prophecy or its principals were allegedly defrauding their investors, nor did I conspire in any fraud," Kahn said in a statement to Reuters.

Bryant Riley, B.Riley's eponymous CEO, said during an analyst earnings call last week that it had invested in Franchise Group based on the fundamentals of the business and that Kahn had no "direct experience with what has been alleged."

B. Riley has also been targeted by a short seller. In February, Wolfpack Research accused the company of not cutting losses on "failing" investments and continuing to extend capital to distressed clients.

"Although B Riley attempted to address short seller concerns on their recent earnings call, the information they provided was limited," said Rick Meckler, partner at Cherry Lane Investments.

"It remains to be seen whether this is a real problem for RILY, but investors and clients sometimes don't wait around for greater clarity," Meckler added.

(Reporting by Chibuike Oguh in New York; additional reporting by Medha Singh and Niket Nishant; editing by Michelle Price and Deepa Babington)

By Chibuike Oguh