The police were given access to Mr. Kelly’s Facebook exchanges, which showed him discussing the plan with John Mary of Hattiesburg, once the co-host of a conservative talk-radio show that Mr. McDaniel previously hosted and regularly appeared on.

According to those exchanges, which were examined by The New York Times, as well as interviews with people briefed on the case, Mr. Mary and Mr. Kelly hoped to propel rumors about the state of Mr. Cochran’s marriage that had been circulated on social media by McDaniel supporters as a kind of subterranean campaign issue. Mr. Kelly and Mr. Mary planned to make a video, but were unsure how to get a current picture of Mr. Cochran’s wife in the nursing home.

Mr. Mayfield did not take part in these exchanges. But he was contacted at one point, apparently by Mr. Mary, and asked to take Ms. Cochran’s picture, since his own mother was in St. Catherine’s. He declined. Instead, according to the message traffic, he agreed to set Mr. Kelly up with someone else — a person who has not been named or charged — who could help Mr. Kelly carry out his plan.

On May 22, six days after Mr. Kelly’s arrest, three police officers arrived at Mr. Mayfield’s law office, searched his computers and led him away in handcuffs.