"Fatal Vision" author Joe McGinniss is expected to return to the witness stand today as a federal court hearing reviews the 1970 murder case that became the subject of his best-selling book.
On Friday, McGinniss testified about how his impression of Jeffrey MacDonald changed from friend to psychopath.
"Psychopaths are very charming people," McGinniss said. "I felt genuine affection for him... It was a tough fight for my heart."
Forty-two years after the murders of MacDonald's pregnant wife and two young daughters, the former Army doctor hopes new forensic science and previously unheard testimony will convince a federal judge to either vacate his 1979 conviction or grant him a new trial.
But McGinniss, who inked a deal with the defense for unrestricted access during the 1979 trial, remains convinced MacDonald is exactly where he needs to be -- in prison.
During his research after the trial, McGinniss said he found a note MacDonald had written to his lawyers, indicating he might have taken diet pills, called Eskatrol Spansules, the night of the murder. The author theorized the medication's stimulant properties might have caused MacDonald to snap.
"It was particularly dangerous in terms of side effects," McGinnis said. "In fact, the manufacturer took it off the market… Common side effects were rage reactions."
During cross-examination, McGinniss conceded he could not prove MacDonald actually took the pills the night of the murder, despite his suspicions.
Throughout this hearing, MacDonald's defense has centered on secondhand accounts of private conversations a now-deceased drug user, Helena Stoeckley, had with her mother and a U.S. marshal (both deceased). In those conversations, Stoeckley allegedly admitted to being at the crime scene.
This would support MacDonald's story that his family was attacked by four drug-crazed intruders.
But when she testified in 1979, Stoeckley denied any recollection of the pre-dawn hours of Feb. 17, 1970, when the crime took place. And McGinniss said a private interview with the defense team produced equally poor results, even after lawyers assured Stoeckley she would not face prosecution for admitting involvement in the crime.
Reading from his book, "Fatal Vision," McGinniss recalled Stoeckley telling the defense team, "Do you realize how much drugs I've taken since that happened? I'm not gonna sit there and say yes to things I didn't say, or things I don't remember saying."
Fox News' Mary Quinn O'Connor contributed to this report.