LANCASTER, Pa. — A nurse has been charged with killing a chronically ill 11-year-old boy six years ago with a lethal dose of morphine.

Joy O'Shea-Woomer, 48, of Lancaster, was charged Tuesday with criminal homicide and two other felonies. Prosecutors said she was substituting for Brent Weaver's regular nurse on Sept. 26, 2002, and had "sole custody" of the boy when he died between 11 p.m. and 6 a.m.

The boy's parents, Mark and Carol Weaver, were sleeping and no one else came in contact with Brent Weaver during those overnight hours in his East Hempfield home, according to a police affidavit.

Morphine had not been prescribed for the boy, whose body was exhumed for autopsy as part of the investigation.

What caused the delay in filing murder charges wasn't clear. District Attorney Craig Stedman said he was unhappy about it, but that he knew nothing about the child's death until a few months ago.

"All we can do is move forward," he said. "We are doing what we believe is right."

Brent Weaver needed constant care for cerebral palsy, seizure disorder and spastic paraplegia, which required him to use a feeding tube, Stedman said. His mother told investigators that the boy was "laughing, happy and acting normal" the night before he died, authorities said.