Hennepin County Attorney Mike Freeman said an autopsy showed Reed had multiple bruises, fractures, bite marks and puncture wounds that all came from the same violent incident.
Freeman said evidence shows Carla Poole, 37, beat and killed Demond Reed in anger, because she said he soiled his pants.
"Poole then made up stories about how someone had allegedly abducted this young man, which turned out to be false, sending police on a wild goose chase," said Freeman. "The death of Demond is tragic in itself. Having that death and beating observed by a 4-year-old, a 6-year-old and an 11-year-old just compounds the tragedy."
Freeman said there was no evidence that Poole had ever seriously abused the children before they witnessed the fatal beating. According to the criminal complaint, the 6-year-old said Poole told him and the 4-year-old to hold Reed down by the arms while she beat him.
An autopsy found multiple bruises on Demond's head, body and extremities; injuries inside his mouth that were consistent with a blunt object being shoved in it; rib fractures; puncture wounds on his stomach, back and buttocks; a large hematoma on his left eye; a bite mark on his stomach; and bleeding inside his brain, the criminal complaint said.
Minneapolis Police Chief Tim Dolan said Reed's death is the city's first homicide of the year. He said the violent nature of the incident is upsetting to the entire department.
"Anytime a child is hurt, anytime you see a child that has been killed in a homicide, it affects the police officers deeply," Dolan said. "As you see in the complaint, it couldn't be any worse then we all imagine."
Demond Reed came to Minneapolis from Chicago around Christmas with his father. Last week his father was ordered to the Hennepin County workhouse on an unrelated complaint, and left his son with his cousin, Carla Poole.
Police found Reed's body early Sunday, after investigators began to doubt Poole's original abduction story.
The boy's body was found in a closet in Poole's home, inside a green canvas bag tied with a white cord. That bag was also inside a plastic garbage bag that police say had Poole's fingerprints on it.
According to the criminal complaint, Poole told her 11-year-old daughter last Wednesday to call Demond's relatives and say he was missing.
Poole told police at the time that a friend named Shawna had taken the boy without her knowledge or consent -- a story authorities now say was a lie.
As alleged by the complaint and authorities, on Saturday the 11-year-old told police her mother had "spanked" Demond after he soiled his pants. The girl said he had a seizure and began vomiting.
She said his health deteriorated over the next several hours, and that Poole discussed calling 911 but decided against it because she was afraid.
Demond then stopped breathing, and Poole's efforts to revive him with CPR failed. The 11-year-old said Poole put a blanket over the boy and left him on a bed for about two days before stashing him in a closet.
Police said a homicide investigator interviewed Poole after advising her of her rights. She initially claimed "Shawna" took Demond, then claimed another child had beaten the boy, and finally confessed, the complaint alleged.
Police Lt. Amelia Huffman said she doesn't know why it took police so long to find Reed's body.
"The home at 3118 Morgan Ave. N. was searched a couple of times during this investigation, including late Friday afternoon with the use of cadaver dogs," said Huffman. "At that point, neither the dogs nor the investigators found any evidence of the presence of the boy."
"There was, however, quite a number of bags and boxes -- just a large amount of stuff in this home, which certainly obstructed both the ability of the investigators, and likely the ability of the dogs, to pick up any scent," Huffman added.
Freeman said Poole's four children were placed in protective custody. He said his office is trying to contact the fathers of Poole's children and is working on an emergency order to terminate her parental rights.
Freeman said Poole's first court appearance is scheduled for 1:30 p.m. Wednesday, and charges could be upgraded to first degree murder.