It started Sunday when Damani Terry was struck and killed by a GMC van after he darted onto the road from the yard where family members were holding the party.
The distraught driver, Archie Brown Jr., 40, immediately stopped and got out to tend to the boy. But it was too late; Damani was dead.
Damani's family came running, including his older brother, 15-year-old Rasheed Chiles.
Soon, two more people would die: the driver and the brother, both felled by bullets fired by the same man.
Brown was targeted, police say; Rasheed was struck by a stray bullet, his family believes.
Police aren't publicly identifying who the shooter was, other than to say he was a relative.
Damani's family members say he was the boys' uncle. And they want him to turn himself in.
Four days later, the man was still eluding authorities Wednesday.
"What did we have Sunday?" Milwaukee Police Chief Ed Flynn asked. "We had some clown take the law into his own hands and murder a guy who was doing what we expect good citizens to do, and oops, accidentally kill somebody else."
'The worst day I ever lived'
"Sunday was the worst day I ever lived in my whole entire life," Lena Tidwell, told CNN affiliate WISN. "It's a day I wish had never, never came."
Tidwell is the boys' grandmother, and the mother of the alleged gunman.
She said she was inside eating cake and ice cream at the birthday when her life turned upside down.
"I didn't know my grand baby got hit, and then I heard people just screaming," she said. "Then I just heard gunshots, and my daughter ran in the house with the baby in her arms."
The family believes the uncle targeted Brown, and Rasheed was struck by an errant bullet.
"I heard he was trying to pick his brother up. He was trying to hold him and save him," a family friend, Stephanie Townsend, told CNN affiliate WITI
'My son didn't mean to hurt your child'
Over at the Brown family household, his father flipped through an album, looking at photos of his son.
Archie Brown Jr., the van driver, was the father of four daughters, the youngest only 6 months old.
"I know my son didn't mean to hurt your child, but your child is gone, your nephew is gone, and my son is gone," Brown's father told affiliate WTMJ
. "The best thing we can do is trust in God to give us strength and make it through this process."
Brown's friends described him as a well-known businessman who didn't hesitate to get down on his hands and knees.
A friend, Kwabena Nixon, recalled how Brown helped out when a night club, in disrepair, was on the brink of closing.
"I remember he got on the roof and literally put the roof together on his own time and out of his own pocket," Nixon told the affiliate.
On the day of the incident, another friend, Homer Blow, said he raced to the scene soon after the accident. He told WITI
that a man came out and told Brown he had just signed his death warrant. The man then fired a gun, Blow said.
"Just to hear how this coward took my friend's life in cold blood for doing the right thing," Blow said.
'It really needs to stop'
Tuesday night, dozens gathered at a vigil to remember Damani and his brother. Pastor Malcolm Hunt, who led prayers at the vigil, says another is planned Wednesday for Brown. He would like to see the two families together there.
"I really feel like in order for us to truly make a statement, both families need to have some kind of reconciliation together," Townsend told reporters at the vigil.
In the neighborhood where Damani died, neighbors lamented how quickly people reach for guns to settle scores.
"I'm glad I didn't accidentally run over no kid, and then some fool come out here and start shooting. That's crazy," resident Percy Watson told WISN
"It really needs to stop."
'Basically assassinated this gentleman'
Mayor Tom Barrett didn't mince words either as he echoed those sentiments.
"Someone got angry, someone took a gun and basically assassinated this gentleman," he said of the death of Brown.
"Three deaths. Three deaths. Started out as a tragedy as an accident, but a total loss of control -- the total inability to control oneself in that setting -- led to two more tragic deaths."
As police search for the alleged gunman, his sister -- and the boys' mother -- Rikeesha Tidwell is making funeral arrangements for her two sons.
"I'm very worried about him, but what can I do?" Rikeesha Tidwell said.
A Milwaukee lawyer has offered a $25,000 reward in the case. But, for now, police are yet to catch up to the gunman.
"Do the right thing," Lena Tidwell said, addressing her son. "If you were involved in that, do the right thing, just give yourself up."