DETROIT, MI -- Sylvia Majewska, 65, killed her 7-month-old granddaughter and slit her son's throat with a box cutter blade Nov. 22, Oakland County sheriff's officials said they believe Monday.

Investigators found the bodies of 29-year-old Daryne Gailey, who's developmentally challenged, and his daughter, Charley Lillian, inside Gailey's Oxford Township home the following morning. It's believed Majewska, described as an "angry woman and a controlling woman," by Oakland County Sheriff Michael Bouchard, committed the killings the night prior but had been planning it prior.

"For whatever reason, she came to the conclusion that the son and daughter would be better off not alive," Bouchard said.

Daryne Gailey was divorced from the baby's mother and lost custody of his child. He was allowed bi-weekly supervised visits, usually monitored by Majewska.

Investigators executed a search warrant at Majewska's residence in Oakland County's Addison Township and found documents that "very clearly shows you, it's ominous, and its a conclusive decision where this is headed," Bouchard said.

One document included instructions letting the recipient know a recently purchased $60 child seat could be returned.

Bouchard said it appears Gailey was attacked in his bed and moved to the bathroom.

Majewska was arraigned from the hospital about 11:30 a.m. Monday on two counts of premeditated murder.

The baby, who died of blunt-force trauma, appeared to have suffered "a soft blow" to the head, said Bouchard. The child had been placed in the crib, "dressed and tucked away like it was going to sleep."

Bouchard, who saw the dead child, described her a "beautiful little baby."

"Anyone looking at this baby would certainly think there are so many other options than this," the sheriff said.

There was clear premeditation "and a plan that things were all going to come to an end shortly," Bouchard said, based on evidence collected during the investigation, including letters and information from Majewska's computer.

Majewska remains hospitalized with lacerations to her arm and has not undergone police interrogation. She is under guard at the hospital and being treated for other medical issues unrelated to the cutting, Bouchard said Monday.

Deputies visited Gailey's home for a wellness check and found the woman on the couch bleeding from her wounds into a pail.

Bouchard said Majewska, a recently retired, educated woman with a master's degree in art, was working at Meijer's. She cut her left arm "pretty significantly" and would have likely died had law enforcement no intervened.

It appeared to be an attempted suicide. Detectives suspected Majewska of the killings from the beginning.

"We believed she was certainly very suspect," Bouchard said. "She was the only one alive."

Investigators quickly ruled out that it was a random incident committed by someone outside the home.

In the last year of Gailey's life, he married Amanda Hendrick, had a baby with her and then divorced.

According to Oakland County sheriff's officials, Gailey's parents had legal control over him and filed for the divorce on his behalf.

Court records obtained by the Detroit Free Press showed a personal protection order was filed on Gailey's behalf against his ex-wife, but lifted in July.

Oakland County Clerk marriage records show the couple married in Tuscola County's Novesta during the fall of 2013. They filed for divorce in early March and an Oakland County judge finalized the divorce earlier this month.

Majewska is scheduled to return to court Dec. 18 at 10:30 a.m. for a preliminary hearing.