DETROIT (AP) — A woman accused of killing her 9-year-old son and burying his body in her Detroit backyard has been charged with murder, a prosecutor said Wednesday.

A shallow grave holding remains was discovered on Jan. 6 while the landlord was preparing the house for new tenants. No details were released at that time although Detroit police acknowledged an investigation was underway.

Zemar King was smothered to death on Oct. 24, two weeks before his 41-year-old mother left for Georgia with a 3-year-old son, Wayne County prosecutor Kym Worthy said.

“I have been a prosecutor for a long time, and I often say that I have seen it all. The horrors of this child abuse case defy that,” Worthy said. “The alleged actions of this defendant are among the most callous I have seen and hope never to see again.”

The mother was arrested Jan. 10 in an unrelated case in Cobb County, Georgia, online records show. She was awaiting extradition to Michigan on a murder charge and other crimes.

Worthy said Zemar’s father recently became aware of the boy’s death.

“This case is rather shocking,” said Todd Bettison, Detroit’s interim police chief. “In my 27-year career, I don’t remember seeing anything like this.”