Gerson Lehrman Group, the largest so-called expert network firm, has been hit by a series of top-level executive departures in recent weeks, according to three sources familiar with the company.

At least a half-dozen top sales executives, including one who managed Gerson Lehrman's relationship with hedge fund customers, have left the New York-based firm, said these sources, who were not authorized to speak to the media.

The departures from Gerson Lehrman come at a time when U.S. authorities have launched a new crackdown on insider trading, which has resulted in the arrest of eight people associated with a rival California-based expert network firm.

No one associated with Gerson Lehrman has been implicated in the investigation.

The sources said there is no indication that any of the departures were directly related to the insider trading investigation.

All of the executives left on their own accord, these sources said.

Expert network firms, in exchange for a fee, specialize in matching hedge funds seeking information with industry consultants--many of whom are moonlighting corporate employees.

The timing of the departures reflects some of the fall-off in demand for expert network services in the wake of the investigation, said one of the sources. But the source added that the demand for expert network services began declining even before news of the insider trading investigation broke.

A spokeswoman for Gerson Lehrman declined to comment.

Another source, who is a person close to Gerson Lehrman, said the departures had nothing to do with the firm losing business over the past few months.

The person, who declined to be identified because he was not authorized to speak to the media, said some people had left the firm for personal reasons and the company began overhauling its sales staff last year. This person said that the firm hired a new national sales manager last year .

The most recent departures from the firm are James Yockey and Ramesh Karnani, both managing directors, according to the sources. Yockey was responsible for bringing in new business from hedge funds. Karnani, meanwhile, sought to forge sales relationships with investment banks. Both left in January, the sources said.

Both Yockey and Karnani were long time employees of Gerson Lehrman. The sources said neither had been replaced and that both executives were relocating for personal reasons.

At least three other top sales executives Aaron Liberman, David Moon and Matthew Calistri have left the firm since late last year, these people said.

Gerson Lehrman, which employs over 700 people, had a 40-person sales staff at its peak.

The news of their departure comes only days after Gerson Lehrman hired a Washington lobbyist firm Elmendorf Ryan to represent its interests.

The expert network business has come under a cloud ever since the government arrested a handful of experts and executives working at rival expert network firm Primary Global Research and charged them with insider trading.

Several of the departing employees have left to join new ventures, relocated to other countries or were recruited by rival expert network firms, these people said. Many of the moves may have been in the works for quite some time, some of these people said.

Private equity firm Silver Lake Partners, which took a $200 million equity stake in Gerson Lehrman in 2007, did not immediately comment on the executive changes at the firm.

(Reporting by Emily Chasan and Svea Herbst-Bayliss; editing by Carol Bishopric)