PITTSBURGH — A Pennsylvania boy who was 11 when he was accused of killing his father's pregnant fiancee with a shotgun blast to the back of her head as she lay in bed will be tried as an adult in the death of both the woman and the fetus, a judge ruled Monday.

Jordan Brown, now 12, is charged with criminal homicide in the death of 26-year-old Kenzie Marie Houk in their farmhouse in New Galilee, in western Pennsylvania, on Feb. 20, 2009. Houk was 8-1/2 months pregnant; the male fetus died from a resulting lack of oxygen.

"This offense was an execution-style killing of a defenseless pregnant young mother. A more horrific crime is difficult to imagine," Lawrence County Judge Dominick Motto wrote in his opinion refusing to move the case to juvenile court.

The boy could be convicted of anything from involuntary manslaughter to first-degree murder under Pennsylvania's homicide law. Prosecutors have said they will seek a conviction on first-degree murder charges, for which he could face up to life in prison if convicted.

The attorney general's office, which is prosecuting the case, and Browns' attorneys did not immediately return calls seeking comment on the ruling.

Prosecutors have suggested the boy was jealous of Houk and her unborn son. Police had said Brown's hid the weapon under a blanket so Houk's 7-year-old daughter wouldn't see it as he entered her mother's room. Later, authorities say, he threw the spent shell casing along a path on his way to a bus and went to school. A state trooper testified that tests showed the shell was fired from Brown's youth-model 20-gauge shotgun.

Brown's attorneys, Dennis Elisco and David Acker, had argued that the boy was too young to realize the full import of his actions and that he would best be dealt with in juvenile court, where he could receive treatment and incarceration specifically aimed at younger offenders.

Under state law, the attorneys had to convince the judge that he was more "amenable" to rehabilitation in the juvenile system — which would have jurisdiction only until he is 21 — than as an adult.

But the judge said the testimony of defense psychologist Kirk Heilbrun didn't convince him that Brown was best tried as a juvenile. Heilbrun said that the boy was likely at low risk of offending again — but the judge noted the assessment did not question whether Brown committed the crime.

Motto focused on findings by a prosecution psychiatrist, Dr. John O'Brien, who found that Brown tended to "minimize" his wrongdoing and to "deny" and "shift blame" for his misdeeds. Brown, specifically, denied killing Houk when examined by both doctors.