CHICAGO (AP) - Chicago prosecutors created a buzz by calling Oscar-winner Jennifer Hudson as their first witness during the trial of the man accused of killing her mother, brother and nephew. But the hard part starts Tuesday, when they'll have to get down to the nitty-gritty of their case.

With no surviving witnesses to the murders, prosecutors must offer overwhelming circumstantial evidence that William Balfour, the ex-husband of Hudson's sister, committed the grisly crime on Oct. 24, 2008. They are expected to introduce evidence in the next few days that includes cellphone records and security-camera footage that place Balfour in the area of the killings, since he denies he was there.

Another challenge will be tying Balfour to the alleged murder weapon, a silver and black .45-calibre handgun that sat Monday on a stack of papers at the prosecution table in plain view of jurors and Hudson, who testified - and attracted a large crowd of national and local media - earlier in the day.

Public defender Amy Thompson told jurors during her opening statement that DNA found on the gun didn't match Balfour, which "absolutely, positively" excludes him as the killer. But prosecutors claim that Balfour targeted the family in a horrific act of vindictiveness against his ex-wife.

Shortly after Thompson and prosecutors laid out their cases, Jennifer Hudson took the witness stand in sometimes tearful, gut-wrenching testimony. Wearing a simple, all-black dress, the singer-actress broke down at one point, dabbing tears as she testified just yards away from Balfour.

The "American Idol" finalist and Oscar winner, who was in Florida at the time of the shootings, spoke of her family and her reaction to her sister, Julia Hudson, telling her in 2006 that she was marrying Balfour.

"None of us wanted her to marry him," the 30-year-old said, her voice cracking as she struggled to hold back tears. Asked later if she was ever friends with Balfour, whom she knew from junior high school, Hudson answered with disgust.

"Never," she said firmly. "I tried to keep my distance from William Balfour."

With her hair up in a bun, Hudson at first seemed composed on the stand - even as she leaned around the judge's bench to identify Balfour. But she began crying when talking about seeing her family for the last time the Sunday before the killings.

After just more than 30 minutes, she stepped down, grabbed a fistful of tissues and walked across the courtroom directly in front of jurors. She took a seat next to her fiance, David Otunga, best known for his stint on VH1's reality show "I Love New York."

Sitting on a fourth-row bench, she bowed her head and wiped away tears after prosecutors called her sister to the stand and began playing a recording of the 911 call Julia Hudson made after discovering their mother's bloodied body.

"Oh my God, oh my God," she is heard yelling at a dispatcher, who tells her to stop screaming because he can't understand her. "My momma, my momma!"

Balfour, who pleaded not guilty to three counts of first-degree murder, slumped in his chair and kept his eyes fixed on his former wife, whose divorce from Balfour was finalized just last year.

Julia Hudson described how Balfour repeatedly threatened her and her family after she rejected his pleas in May 2008.

"He said, 'If you leave me, you will be the last to die. I'll kill your family first,'" she said, her voice quivering.

Under cross-examination, she acknowledged she was still having sex with Balfour days before the slayings.

The killings happened the day after her birthday. Prosecutors believe that Balfour became enraged by balloons he saw at the home that he thought were from her new boyfriend.

Thompson suggested during her opening that the killings may have been a result of alleged drug dealing by Hudson's brother, Jason Hudson, in the crime-ridden South Side neighborhood where they lived in Chicago. She claimed that police pinned the slayings on Balfour because they felt pressured to make an arrest, since the victims were a celebrity's relatives.

"The police were on the hook," she said. "They had to find their man and find him fast."

Prosecutors said Balfour went inside the three-story house around 9 a.m. and shot Hudson's mother, 57-year-old Darnell Donerson, in the living room, then shot her 29-year-old brother, Jason Hudson, twice in the head as he lay in bed.

Investigators allege he then drove off in Jason Hudson's sport utility vehicle with 7-year-old Julian inside, and later shot the boy in the head as he lay behind a front seat.

If convicted of at least two of the murder counts, Balfour would face a mandatory life sentence.

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