Nearly four years ago, 6-month-old Larry Lord died at Northern Light Eastern Maine Medical Center after being transferred from a Millinocket hospital.
According to the prosecutor, Jessee Mackin shared a Millinocket home with the infant, the boy's sister, and the baby's mother, Mackin's girlfriend of two months.
Superior Court Justice William Anderson will decide Mackin's guilt or innocence in the jury-waived trial.
Assistant Attorney General Meg Elam said that at one point on the afternoon of May 5, 2015, Mackin changed the baby's diaper and that while in Mackin's arms, the baby stiffened and went limp. The infant was never able to breathe on his own again, Elam said.
In her opening statement, the prosecutor told the judge that Mackin was the only person who had the opportunity to inflict the injury causing the infant's death.
Mackin's defense attorney, Stephen Smith, said the accused will testify and tell the judge he did not cause Larry Lord's death.
"He doesn't know who did it. But he knows who else was there. And the court will hear testimony about that," Smith said.
Elam, however, told the judge, "When you determine the window of time when Larry was injured, you know who injured him."
Mackin has been out on bail since shortly after his arrest in 2016.
The trial is expected to last through the week.