A Damascus father and former Marine pleaded guilty Monday to fatally beating his adopted 3-year-old son, with whom he said he'd never formed a bond.

Brian O'Callaghan, 37, who served in Iraq, was indicted in March 2014 on charges of first-degree murder and first-degree child abuse resulting in the death of his son, Hyunsu. A judge ordered that O'Callaghan be held without bond until he is sentenced in April. He faces a maximum sentence of 40 years in prison for the boy's death, WTOP reports.

Montgomery County prosecutors dropped a first-degree murder charge against O'Callaghan – which carries a life sentence -- as part of a plea deal. O'Callaghan had worked for the National Security Agency.

Investigators were called to Children's National Medical Center in Washington on Feb. 2, 2014, and told the boy's injuries, including trauma to the brain, indicated he had been beaten, WJLA reported. The child died the next day and police say an autopsy determined the manner of death was homicide by multiple blunt impact injuries. Hyunsu had skull fractures, bleeding in his brain and hemorrhaging in his eyes when he arrived at the hospital and doctors were concerned that he was "brain dead," WTOP reports.

O'Callaghan told police that his son slipped in the shower. But investigators say a typical child's fall would not have caused Hyunsu's fatal injuries.