The .45-caliber gun was discovered Wednesday in a vacant lot in the West Side neighborhood where the body of Hudson's 7-year-old nephew Julian King was found a few days earlier inside an SUV. The bodies of Hudson's 57-year-old mother Darnell Donerson and her 29-year-old brother Jason Hudson were discovered in the family's home last Friday.
"The weapon has been positively identified through forensic examination as the weapon that was used in the homicides," police Superintendent Jody Weis said at an unrelated Friday news conference.
Police were "extraordinarily pleased and satisfied" the weapon had been identified, but much work remained in the case, Weis said.
A police official has said detectives think Hudson's nephew was alive when he left the house where his relatives were killed and likely was shot in the SUV where his body was found. The police official, speaking on condition of anonymity because the official was not authorized to discuss the case publicly, would not elaborate on when detectives think the boy might have died.
Nobody has been charged in the killings, but 27-year-old William Balfour, the estranged husband of Hudson's sister, is in custody on a parole violation. Police have called Balfour, a convicted felon, a "person of interest."
News linking the gun to the killings comes as the South Side church where Hudson spent her childhood prepares for a weekend memorial service honoring the three slaying victims. Pleasant Gift Missionary Baptist Church is also where Hudson's sister Julia made an emotional plea for her son Julian's return the day after he disappeared and his relatives were found shot to death.
The Hudson sisters will not attend the community memorial, which will be open to the public from 4 p.m. to 7 p.m. Sunday, according to an announcement from Hudson's publicist.
Funeral services for Hudson's mother, brother and nephew are to be held Monday. The private service will be held at Apostolic Church of God on Chicago's South Side.
To help other victims of violent crime, the Hudson sisters have established the Hudson-King Foundation.
Associated Press Writer Don Babwin contributed to this report.