In it, Mr. Byrne said he had helped the F.B.I. with the “Clinton Investigation” and “Russia Investigation.” Those, he said, “turned out to be less about law enforcement and more about political espionage conducted against Hillary Clinton and Donald Trump.”

Although the statement did not mention her directly, it referred to what he described as a romantic relationship with Ms. Butina, who is serving an 18-month prison term after being accused by federal prosecutors of trying to infiltrate powerful political circles at the direction of the Russian government. She ultimately pleaded guilty to a lesser charge.

Mr. Byrne said he met Ms. Butina at a libertarian convention in Las Vegas in 2015. Over the course of their relationship, he said, she spoke increasingly about meeting or seeking to meet people involved in the presidential campaigns of Ms. Clinton, Mr. Trump and others. That, he said, had made him wary. He eventually began communicating with the F.B.I. about his interactions with her.

Mr. Byrne said he had released the statement because he realized that someone would uncover his involvement in the federal investigation if he did not reveal it himself. He decided to come forward after a conversation with his “Omaha rabbi” — a reference, he said, to Mr. Buffett.

Mr. Byrne grew close to Mr. Buffett through his father, John Byrne, who led GEICO’s financial turnaround in the late 1970s. Berkshire Hathaway invested in and later acquired the insurer. Mr. Buffett later tapped the younger Mr. Byrne to run another Berkshire Hathaway company that made uniforms for firefighters, police officers, nurses and others.



Mr. Byrne bought a little-known retailer in 1999 and transformed it into Overstock.com, buying up inventory from the many start-ups that went bankrupt in the dot-com bust. But shortly after the company went public in 2002, its stock plunged.

Mr. Byrne blamed a vast network of short sellers, which caused a family rift: His father eventually left the board, saying he could not support his son’s crusade against Wall Street.