(CNN) Sheriff's deputies found a grisly and heartbreaking scene when they answered a 911 call at a Memphis apartment complex, authorities said Saturday.

Four children, ages 4 years to 6 months, had been stabbed to death, Shelby County Sheriff Bill Oldham said.

The children's mother, Shaynthia Gardner, 29, was arrested at the scene and is accused of killing her children, Oldham said. Investigators are trying to determine a motive..

"One of the most difficult questions in any investigation always is, why did it occur?" Oldham said, according to CNN affiliate WMC.

A fifth child, a 7-year-old boy, escaped the mayhem, Oldham said.

Read More