Historical accuracy
The story Boston Strangler presents is largely historically accurate, including the details of the crimes and investigation, the names of the victims, the sexism Loretta and Jean faced, and the fact that Loretta and Jean coined the name Boston Strangler. That said, authorities, authors and others still disagree about whether DeSalvo was the sole killer or whether the murders could have been the work of multiple individuals, as posited by the film.
Director Matt Ruskin interviewed the children of Loretta and Jean as well as their former colleagues to learn more about them and their work. In addition, Ruskin read all of Loretta and Jean's articles and "some of the headlines were pulled right from the paper. Keira has some voiceover in the film, where you hear what she's actually writing, and some of that was pulled directly from their stories, too".
Several historical inaccuracies stem from condensing the timeline. For example, in the film Loretta begins investigating after three murders, while in real life she did so after four. Sophie Clark is murdered on New Year's Eve in the film but in reality, she was discovered on December 5. In the film, the copycat murders in Michigan occur in 1965 but they actually occurred between 1967 and 1969. The film shows DeSalvo's death as occurring shortly after his conviction. In reality, the Boston Strangler's final murder was in 1964, DeSalvo was convicted in 1967, and he was killed in 1973.
Other major details changed for the film include:
The character Detective Conley is a composite of multiple real detectives who worked on the case.
The film shows Loretta and Jean meeting for the first time shortly before working together on the stories in 1962, in real life, they worked together as early as April 1952.
In the film, DeSalvo calls Loretta and tells her to come to visiting hours the next day to hear his side of the story; he is killed by other prisoners that night. In real life, DeSalvo called psychiatrist Dr. Amos Robey just before his death.
Daniel Marsh is actually a pseudonym given to a suspect in the case whose identity has never been made public; a suspect also referred to as David Parker by some media. In the film and in real life, he was a former Harvard student with a history of domestic abuse who moved to Michigan before copycat murders took place. In the film, the suspect's real name is given as Daniel Marsh, and it does not mention that another suspect was later convicted of one of the murders in the Michigan case.
While the character of Paul Dempsey is fictional, some of the details about his life are based on suspected serial killer Charles E. Terry.
