Karen Read pleaded not guilty to murder charges in the death of Boston police officer John O’Keefe and is facing a retrial after a jury was unable to reach a verdict last year.

On Monday morning, the jury heard detailed and technical testimony about cellphone data, which the prosecution is using to try to establish John O'Keefe’s location before he was found unconscious in the snow outside the Canton home.

Cellebrite expert Ian Whiffin walked jurors through one of the most contentious pieces of evidence in Karen Read’s case Monday.