A federal criminal investigation into short sellers has spawned a behind-the-scenes feud between a law professor whose research is helping to guide prosecutors and one of the most prominent investors in their sights.

Carson Block, known for public broadsides against companies he suspects of fraud, has been waging a campaign to discredit Joshua Mitts, a Columbia University law professor who has been aiding the US Justice Department in its investigation into whether short sellers use illegal trading tactics to drive stock prices down.