"In terms of what he has said so far, it may be borderline manipulative, but it does not yet appear to be stock manipulation because a lot of the statements he's making are general statements, but they are conclusory," Frenkel said in an interview on CNBC's "Squawk Box."
The FBI and federal prosecutors are investigating whether Herbalife's stock has been manipulated, The Wall Street Journal reported Thursday.
Sources told the publication the authorities have interviewed people connected to Ackman, who shorted Herbalife's stock and waged a public battle with the company over the course of more than two years, alleging the nutritional supplement distributor is a pyramid scheme and insisting the stock price will go to zero.
"I'm puzzled by the story. We have not heard from the FBI or Department of Justice," Ackman told CNBC. "If they have any questions for me I'm happy to answer them. I stand by every statement I've made about Herbalife. The company has never identified anything we've said that is false."
Ackman took a short position against Herbalife in May 2012, and went public with it the following December.
What followed was a contentious period during which Ackman came under fire from Herbalife and other well-known investors, including Third Point founder Dan Loeb and billionaire investor Carl Icahn. Ackman aggressively defended his position—including during an on-air battle with Icahn during CNBC's "Fast Money: Halftime Report."
In February, the Pershing Square Capital Management founder told CNBC he was through with disclosing his short positions to the public.
Frenkel said Ackman's campaign against Herbalife was notable for its highly public profile.
The real quality short sellers do a lot of this work behind the scenes," he said. "They're trying to get their message to the government to understand their analysis to understand why the company is not as strong as the company wants the public to believe."