Carson Block, the short-seller who founded research firm Muddy Waters, on Friday challenged his latest target, Chinese mobile security software company NQ Mobile, to sue him in the U.S.
NQ Mobile's U.S.-listed shares have fallen just over 50 percent since Muddy Waters accused the Chinese firm of fraud last month. The shares closed Thursday at $10.97.
"NQ has sued me in China, which I think a lot of us can admit is basically a joke for them to do that. If they want to hold me responsible, if they feel there is something not right, come and sue me in the U.S.," Block told CNBC Asia's "Squawk Box."
A representative of NQ told CNBC he was unable to comment on any current or potential legal activity.
"Muddy Waters has no known address or phone number; its principal, Carson Block, refuses to disclose where he lives," mobile-security software provider NQ Mobile said in an emailed statement to CNBC prior to the Block interview.
Muddy Waters is a high-profile short-seller, with a long history of targeting companies, mostly Chinese firms listed in the U.S. or Canada, which it claims are fraudulent. In 2011 it released a damaging report accusing Sino-Forest, a Chinese company then listed in Canada, of committing fraud. Sino-Forest's shares fell sharply as a result and the company filed for bankruptcy last year.
Short-sellers profit by borrowing shares they expect will fall and buying them back at a lower price.
A report by Muddy Waters on October 24, said, "NQ's largest customer by far is really NQ." The short seller also alleged NQ's cash balances likely don't exist. Since then, it has followed up with other reports on the company.
In an October 25 press release, NQ provided details of its bank deposits in response to the allegations about its cash balances and said the Muddy Waters accusations were "false and inaccurate and contain numerous errors of facts, misleading speculations and malicious interpretations of events."
Jim Oberweis of Oberweis Asset Management, an institutional investor in NQ, told CNBC he had personally confirmed the cash deposits NQ holds at Standard Chartered Bank in Beijing following authorization from the Chinese firm to do so.
The cash balances were also confirmed by Taek-Geun Kwon, founder, managing member of Toro Investment Partners, which holds 1.5 million NQ American Depositary Shares.
"We're proceeding down the line to check on every other major source of business for the company," Kwon told CNBC, noting he was in Beijing to conduct a three-week-long due-diligence expedition to examine all of Muddy Waters' allegations. "We're into week one and so far we have not found anything they've gotten right," he said, noting his company has spoken with some of China's largest handset makers, NQ's payment providers and carriers China Mobile and China Unicom, as well as NQ consumers.
"With regards to the Muddy Waters allegations, the first thing is we have to keep the source in mind," Kwon said. "It's not a benevolent organization out there to do social good. It's out there to take short positions and make money for their clients."
He added that NQ had been vetted by auditors, investors and investment banks recently.
Block told CNBC that his team has established its "bona fides" in finding fraud in China, noting his company's previous short-selling successes, such as Sino-Forest, and saying that of the first five companies Muddy Waters accused of fraud, four have been delisted.
"We have people who are former auditors on our team. We have people who are software and tech entrepreneurs on our team. We have the traditional finance guys who you know sit and crank spreadsheets. We have people like myself who are ex-lawyers," he said.
Toro Investment Partners' Kwon said: "If you look at their [Muddy Waters'] track record recently, they've gotten the past seven calls wrong."
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NQ meanwhile has challenged Muddy Waters' sources, saying that the firm does not disclose who its researchers are and what documents they examine.
"I will absolutely not give out their names," Block said. "We've had people picked off in China in the past by the Ministry of State Security. Some of our people have been messed with very hard by the MSS."
— By CNBC's Leslie Shaffer; Follow her on Twitter @LeslieShaffer1