The car, a Chevrolet Suburban, had been stolen from the house on the day of the alleged attack. Julian's step-father, William Balfour, who had become estranged from the boy's mother, has been in custody since Friday. He has denied involvement in the killings has reportedly stopped talking to investigators. On Sunday, Hudson, who won an Oscar for her role in Dreamgirls, had offered a $100,000 (£65,000) reward for the boy's safe return after her sister made an emotional public plea for her son's life.
Balfour, a baker who has convictions for attempted murder and carjacking, had fallen out with Hudson's family after being asked to move out of the family home last winter. Hudson, who rose to fame after competing in the television talent show American Idol, has often credited her success with her upbringing in the impoverished Chicago neighbourhood of Englewood. She had however, encouraged her mother to move away from the crime-ridden area. Miss Donerson refused, saying she did not want her daughter's fame to change her life.
An autopsy showed that Miss Donerson, 59, and Mr Hudson, 29, died from multiple gunshot wounds. The latter's large white four-wheel drive car remains missing.
Neighbours had heard gunshots on Friday morning but were so used to the sound that they ignored them. Police say they are treating the murders as a "domestic" crime. They now believe the shootings began when a gunman fired through a door and hit Mr Hudson.
The attacker then went into the house and continued to fire, hitting Miss Donerson.
Family friends said Balfour and his wife had recently been arguing over ownership of her car and that Balfour had threatened to take the boy away. Balfour's alibi for the day of the killings has reportedly been contradicted by his girlfriend and that police were able to track his whereabouts on the day through his mobile phone.
