CONWAY, Ark. — A woman who adopted her granddaughter after her son's death in Iraq stabbed the 7-year-old girl in the chest, then set fire to their home in an apparent murder-suicide, authorities said Monday.

Janice Robbins, 63, left a suicide note in her pickup truck before killing her granddaughter, Abby Robbins, and herself on Saturday, Faulkner County sheriff's spokesman Maj. Andy Shock said. In the note, she wrote that she didn't want to leave the girl behind.

Robbins stabbed the girl once in the chest had no wounds herself. The coroner determined that the both were killed by the smoke or fire, after finding soot in the girl's lungs that indicated she was breathing while the home burned, officials said.

Abby's father, Army Staff Sgt. William T. "Terry" Robbins, was shot and killed by a fellow soldier in Iraq in 2005 during a dispute over alcohol. Shock said Janice Robbins had another son who had also died.

Janice Robbins had adopted Abby, with the process becoming final in 2010.

At a news conference Monday, officials said there were no indications Robbins was in financial trouble, and investigators were trying to find out if she had been getting counseling for depression. They said the results of toxicological testing, which would show if she was taking medication, will take weeks.

Shock said it appears Robbins planned her and her granddaughter's deaths very deliberately. She left the suicide note in the pickup truck, and taped an envelope to the side of the truck with keys so authorities could get inside, he said. She also left a plastic bin in the truck that contained her will and other personal effects.

"Seems to me that this was not an overnight reaction," Shock said.

Authorities said the deaths surprised people who knew the family. Neighbors told investigators that Robbins and her granddaughter were seen walking dogs earlier in the week.