Under Armour Inc. (UAA) short sellers are "back in the black" after shares in the company plunged over 18% on Monday and turned negative for the year, according to a new note from S3 Partners.

Investors betting against the company's Class A shares saw a single day mark-to-market paper profit of over $130 million yesterday, which wiped out their year-to-date loss of $79.7 million. Those who shorted Class C shares also saw their 2019 loss of $28.07 million erased by a single day paper gain of over $30 million.

The stock slid after The Wall Street Journal reported, and the firm confirmed, that the Justice Department and the Securities and Exchange Commission are investigating its accounting practices. The company also reported better-than-expected third quarter earnings per share and revenue yesterday.

Analyst Sam Poser of Susquehanna Financial Group said, "Failure of management, on the earnings call to address why the investigation was not disclosed prior to last night is disconcerting," according to MarketWatch. “We expect the overhand of the federal investigations, Steph Curry’s injury, and Kawhi Leonard’s emergence with New Balance to cast shadows over Under Armour’s results and stock performance for quite some time."

Before the markets opened on Monday, short interest in the company's Class A shares was $819.8 million, which is 38.78 million shares short or 20.72% of float (actively traded). Short interest in Class C shares was $200.3 million, which is 10.59 million shares short or 5.53% of float. The Class A stock is the second-biggest U.S.-listed equity short in the Apparel, Accessories & Luxury sub-industry behind Hanesbrands Inc. (HBI).

"If one were to consolidate both A & C class listings, Under Armour would take the top spot in the S3 league table for most shorted U.S.-listed Apparel, Accessories & Luxury company," said Matthew Unterman, a director at S3 Partners. "One would only expect this lead to widen if more negative headlines come about after the Federal investigation."