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.

Read is accused of killing O’Keefe, her boyfriend and a Boston police officer, with her vehicle and leaving him in the snow outside a house in Canton, about 20 miles south of Boston. Her first murder trial ended in a mistrial last year after the jury said it could not reach a unanimous verdict.