36-year-old Jessee Mackin took the stand in his own defense Thursday afternoon.
Under questioning from his attorney, Mackin said he didn't know who killed the infant son of his ex-girlfriend.
He also said he never harmed the child.
During closing arguments, prosecutor Leanne Robbin told Judge William Anderson the medical testimony supported the case against Mackin.
"The evidence compels the conclusion that Jessee Mackin caused the death of his girlfriend's baby, Larry Lord," said Asst. Attorney General Leanne Robbin.
Thursday morning, Larry Lord's mother Jamie Clark adamantly denied harming her son.
According to medical testimony during the trial, the baby died of swelling caused by complex fractures to his skull.
The defense attorney pointed a finger at Clark.
In his closing argument, Stephen Smith told the judge the mother had a motive to kill the baby because he was a constant reminder of the abusive relationship she had ended with the child's father.
The baby's father was Anthony Lord, who has since been convicted of two murders and is serving life sentences at the Maine State Prison.
"She wanted to move forward with the new relationship and wipe any memory of Mr. Lord from her consciousness," said defense attorney Stephen Smith. "And I'm not saying that's a rational choice."
Wednesday, a child abuse specialist testified the injuries were so severe they would have triggered an instant coma.
Judge Anderson asked Robbin if the prosecution had a theory as to how the injury happened.
She picked up the baby's bouncy chair and demonstrated.
The judge asked Smith to explain the medical opinions in light of Mackin's denial.
"The doctors weren't there," said Smith. "The doctors say it was lights out. It was immediate. I don't think they were there with a stop watch."
Superior Court Justice William Anderson must now decide whether he thinks Jessee Mackin is innocent or guilty in connection with the death of six-month-old Larry Lord.