Police say the boys were playing "cops and robbers" when the older boy grabbed a gun that was wrapped in pajama pants and stowed on top of a refrigerator. The gun went off, hitting the toddler in the head.
"I never knew that there was a gun in the house," said LaSalle, who lives upstairs from the family.
But he soon discovered that the popping sound was gunfire. Downstairs, he found his 3-year-old grandson Eian lying in a pool of blood.
Clutching Eian in his arms, the terrified grandfather took off running from his house to an emergency room steps away. But the wound was too grave.
Eian was pronounced dead. Now the boys' father, 25-year-old Michael Santiago, faces felony child endangerment charges. He was released from jail Monday after posting bail, CNN affiliate WGN reported
. Authorities say the charges against him could grow.
Police say that during an interview, Santiago confessed to purchasing the weapon illegally from a gang member. He told authorities he kept it for protection.
According to the state atorney's office, Santiago told investigators that about a week ago, he showed the 6-year-old the gun and explained to him that it was only to be used by adults.
At the time of the shooting Saturday, Santiago was at work and the boys' mother was at the store.
On Monday, their grieving grandfather told CNN he has a message now for anyone who will listen.
"They need to keep weapons out of their house," he said.
How frequently are charges brought?
Such shootings are rare -- children are about 19 times as likely to die in traffic accidents and nine times as likely to drown as they are to be accidentally shot, according to federal statistics.
But the shocking nature of such deaths, the tender age of the victims and the national argument over guns combine to create a volatile mixture sure to inflame passions on both sides of the gun control debate.
Gun control advocates argue research shows children often know where weapons are hidden and say weak state laws offer too little deterrent to careless storage of guns. The National Rifle Association has argued that mandatory storage laws are unnecessary.
According to the Centers for Disease Control and Prevention, 69 children under the age of 14 died from accidental firearms discharges in 2013. Of those, 30 were younger than 5.
The pro-gun control group Everytown for Gun Safety argues such deaths are undercounted, saying its analysis of publicly available reports of firearms deaths in 2013 found at least 100 such deaths that year.
Other cases involve children accidentally shooting adults, such as the 2013 death of a woman whose 4-year-old nephew accidentally killed her with a gun owned by her husband and a former soldier who died in Phoenix after being accidentally shot by his 4-year-old son, according to Everytown for Gun Safety.
According to the group's analysis, prosecutors brought charges in at least 88% of shooting incidents involving children and illegally purchased guns -- such as the Illinois case. In shootings where a legally owned weapon was involved in the accidental death of a child, charges were brought in at least 29% of cases, according to the group.
Everytown for Gun Safety researchers said they were unable to determine whether charges were brought in 23% of cases involving legally owned weapons and 6% involving illegally acquired guns.
According to the Law Center to Prevent Gun Violence, 27 states and the District of Columbia have laws that impose varying levels of criminal liability on gun owners who fail to prevent unauthorized access to firearms by children.
Although Illinois is one of those states, Santiago was charged with child endangerment. Prosecutors will need to prove that Santiago knowingly created a condition that could have resulted in harm to his children, CNN legal analyst Joey Jackson said.
While prosecutors typically do not like to charge parents who have lost a child to gun violence -- they have already suffered an unimaginable loss, he said -- in this case, Chicago authorities decided "apparently, you have to draw the line."
"There has to be some deterrent value," he said.
According to Everytown for Gun Safety -- which counts 28 states with child gun access laws -- 14 states don't impose criminal penalties for "mere careless storage." On the other end of the spectrum, laws in three states -- California, Minnesota and Massachusetts -- and the District of Columbia hold gun owners accountable, even in cases where a child may be merely likely to gain access to a carelessly stored gun.
In other states, a child's handling of a gun must cause some harm before laws kick in, the group says.
"Easy access to unsecured firearms is a deciding factor in a majority of unintentional child gun deaths," the group says on its website. "These tragedies are entirely preventable, but many states have yet to adopt policies that would make it harder for children to access negligently stored firearms."
But efforts to expand such laws have been met with resistance.
The NRA, for instance, has argued that state reckless endangerment laws are adequate to combat what it says are "all-time low" rates of gun accidents. The group also argues that universal requirements don't provide enough latitude for gun owners, many of whom may not have children at home, and can prevent people who keep guns for self-defense from using them in a pinch.
The gun rights organization also argues that laws aren't likely to reform careless gun owners and could lead to civil liberties abuses, such as arbitrary storage requirements and police searches of homes to determine compliance.
Regardless, in the aftermath of Saturday's shooting in Chicago, LaSalle -- the grandfather of the dead 3-year-old -- encapsulated what many are likely feeling in the wake of this most recent death: "You know how kids are, they get into everything," LaSalle told WLS. "There is not a safe place where you can put a gun where a kid can't reach it or find it."