The shooting happened Sunday night at 52 E. 103rd Street on the city's South Side.
The gun was registered to a family member in the home, but it was locked up. The boys found a key to access it.
Police officials say Michael Pierce's 14-year-old brother remains in police custody as investigators continue to look into the incident. And, although the Cook County medical examiner has ruled the boy's death a homicide, it is possible the teen will not be charged in connection with the shooting because it appears to be accidental.
"Michael was a blessing to us for 12 years," said Yolanda Pierce, victim's sister.
Despite her grief, Yolanda Pierce shared her memories Monday of a child taken to soon after it appears he was accidently shot to death by his teenaged brother as the two allegedly played with a shotgun.
"Everybody's still in shock. An accident. We don't know, I don't know what happened over there," said Willie Weston, victim's neighbor.
Michael's mother says she found her son at the bottom of the basement stairs in the family's home late Sunday night with massive trauma to his head. The boy was rushed to an area hospital where he died.
Michael's death is a shock to those at the small Christian school he attended.
"You bond with students and you get to know them. It's just difficult," said Jim van Zyl, Roseland Christian school principal.
As news spread, parents of other students -- like Tammy Mastin, whose son played soccer with the victim -- rushed to the school to comfort her son.
"He's pretty hysterical, and a teacher took the phone because he really couldn't even talk to me," said Mastin.
Police still investigating the incident say the boy's 14-year-old brother told them that they had been playing outside when Michael ran inside. Just moments later, the brother says, he heard a loud noise.
A source close to the investigation says the weapon, which was registered to another relative, was locked up but that the boys found a key that allowed them to access the shotgun the family kept in the home for protection.
"How he got to the gun, I couldn't tell you, because it was under three locks. it was under three locks. So how he got to it, we don't know," said Pierce.
The Illinois Department of Children and Family Services has joined the probe into the 12-year-old's death. DCFS says the boy was adopted into family in 1999 out of foster care.
Monday, as Michael Pierce was remembered as a kind and loving child, community activist Andrew Holmes issued a plea to anyone with children and guns in their home.
"Clean your house. If not, I've said it over and over again, we're going to lose another child," Holmes sad.
DCFS says there are two other children in the home who were also adopted out of foster care. The spokesperson says the household has never been a part of any previous investigation initiated by the agency.