Stephen Romero, 6, was killed during the shooting in Gilroy, California, said Gilroy City Councilmember Fred Tovar.
"My thoughts and prayers are with the families of those who (lost) their lives, and those recovering in the hospital. I am deeply saddened by the news of Stephen Romero. I pray that God will grant his family strength. My most sincere condolences. I will keep your family close in my thoughts and prayers in the coming weeks as you are going through your process of grieving," Tovar said in a statement on Sunday night.
Stephen's grandmother, Maribel Romero, told CNN affiliate KGO-TV that Stephen had already been declared dead by the time she reached the hospital.
"I just hope there is justice and they catch the last person or the other person and that there's justice. I want justice for my grandson," Romero told KGO-TV.
Police are still investigating whether there was a second individual involved with the shooting, which left three people dead and 11 more injured.
Romero said her daughter-in-law and Stephen's other grandmother were also shot and hospitalized.