Questions about how to apply securities law to activist investors could complicate any potential insider trading case against billionaire Carl Icahn, pro-golfer Phil Mickelson and Las Vegas gambler William Walters, legal experts said.
U.S. federal investigators are looking into whether Mickelson and Walters may have traded illegally on private information provided by hedge fund manager Icahn, a source familiar with the matter said on Friday. None of the three men has been accused of any wrongdoing, the source said.
Read MoreIcahn, Mickelson trading probe slow to take shape
Icahn's style of investing is to aggressively buy stock with the aim of changing the direction of corporate boards, which makes him an outsider when federal insider trading laws have traditionally focused on corporate insiders, according to legal experts.