"We need to find that kid by sundown," Ziff Sistrunk, a friend of the Hudson family, said Saturday.
Because of the possibility the boy may have been taken across state lines, police spokesman John Mirabelli said the FBI has been asked to assist in the search. FBI spokesman Ross Rice confirmed the agency was helping to look for the child, but he declined to provide details.
Neighbors, friends and relatives gathered Saturday outside the three-story white house on the city's South Side where Hudson went to school and sang in church. Some left stuffed animals and flowers, and others planned a search for Julian.
Members of Hudson's family were expected to make a statement later Saturday.
A family member entering Donerson's home Friday afternoon found her shot on the living room floor. Officers later found Hudson shot in the bedroom, police said.
At least one of the victims suffered defensive wounds, said authorities who described the shooting as domestic violence.
The Cook County medical examiner's office said Saturday that Donerson and Jason Hudson died of gunshot wounds. Their deaths were ruled homicides.
William Balfour, a man suspected in the deaths, was arrested Friday but had not been charged, law enforcement sources told the Chicago Tribune and Chicago Sun-Times.
Police spokeswoman Monique Bond, who declined to comment Saturday on a suspect, said no one had been charged.
Bond said investigators were talking to "a number of people in custody," but she declined to elaborate. An Amber Alert issued statewide Friday, which remained in effect Saturday, said Balfour was a suspect in the double homicide.
Records from the Illinois Department of Corrections show Balfour, 27, is on parole and spent nearly seven years in prison for attempted murder, vehicular hijacking and possessing a stolen vehicle. Public records show one of Balfour's addresses as the home where Donerson and Jason Hudson were shot.
Balfour's mother, Michele Balfour, said that her son had been married to Hudson's sister, Julia Hudson, for several years, but that they were separated. She also said Donerson had ordered him to move out of the family's home last winter.
Family friends said King is Julia Hudson's son, but authorities would not confirm that.
Jennifer Hudson's personal publicist, Lisa Kasteler, said the family wanted privacy. She did not return messages seeking comment Saturday.
The tragedy comes as the 27-year-old Jennifer Hudson continues to reach new heights in her career. Her song "Spotlight" is No. 1 on Billboard's Hot R&B/Hip-Hop charts, and her recently released, self-titled debut album has been a top seller. She was featured in this year's blockbuster "Sex and the City" movie and is also starring in the hit film "The Secret Life of Bees."
She won an Academy Award for best supporting actress in 2007 for her role in "Dreamgirls." In an interview last year with Vogue, Hudson credited her mother with encouraging her to audition for "American Idol," which launched her career.
The singer, whose father died when she was a teenager, described herself as very close to her family. In a recent AP interview she said her family helped keep her grounded.
"My faith in God and my family, they're very realistic and very normal, they're not into the whole limelight kind of thing, so when I go home to Chicago that's just another place that's home," she said. "I stand in line with everybody else, or, when I go home to my mom I'm just Jennifer, (so she says), 'You get up and you take care of your own stuff.' And I love that; I don't like when people tell you everything you want to hear, I want to hear the truth, you know what I mean."
Hudson recently announced her engagement to David Otunga, best known for his stint on VH1's reality show "I Love New York."
Hudson's representatives would not disclose her whereabouts Friday. She had been scheduled to appear Monday in Los Angeles to collect an ensemble cast honor at the Hollywood Awards for "The Secret Life of Bees" with co-stars including Alicia Keys, Queen Latifah and Dakota Fanning.
