Barb Poelman told The Associated Press she was sitting on her deck in the Pinebrook Village mobile home park when "we heard the kids run across the front ... screaming. He (Michael) ran with the kids that were with him."
She said the boy's mother was distraught, pleading for help as she lay on the grass outside the family's home.
"She was screaming, 'Where is the ambulance?' while her boy was laying on the porch bleeding," Poelman said. "The kids, I thought they were just playing."
Witnesses told investigators that four children were playing when one of them, for unknown reasons, pulled a knife and stabbed the boy.
Glen Stacy, who lives nearby, told The Grand Rapids Press that the older boy approached him after the attack and asked to use his phone. He said the boy called 911 to report the attack and turn himself in.
Stacy said he also called police and described the boy as "very calm."
"The only time he raised his voice was when the police came," Stacy said.
When police arrived at the mobile home park, they first went to aid the stabbing victim, but the older boy wanted the officers to pick him up and yelled, "'Hello, I'm right here. You're going the wrong way,'" Stacy said.
The motive in the attack was still being investigated, said Kentwood Police Chief Thomas Hillen.
Police said the 12-year-old boy was evaluated at a hospital then taken to the Kent County Juvenile Detention Center pending charges. The county prosecutor's office will determine whether the boy will be charged as a juvenile or an adult, Hillen said.
A hearing for the boy was scheduled Tuesday afternoon in Kent County Family Court.
Tiffany Armijo said her son was playing with Michael during the attack, adding that she had never seen the 12-year-old before. She said Michael was "a good kid" and her son's best friend.
"He was playing with Michael and his brother, and my kid, too," she said. "They always played together. He was at my house almost every day."
Poelman said the boy lived with his parents and two siblings in the mobile home park. A man who identified himself as Michael's grandfather when the AP called a phone number listed for his home declined to comment.