Deerfield Police Chief John Sliozis says the divorced mother of three came with an alibi.
"She was home sleeping -- went to bed early that night, got up later the next morning, the actual time of the homicide," Sliozis said of Yang's alleged agenda. Yang went on to say that she was home during the entire series of events that police were investigating.
Yang, an aspiring fitness model who worked in real estate, told police she was home on the Northwest Side of Chicago -- 18 miles from the crime scene -- with her children at the time of the murder.
Sliozis said that officers talked to the children about the alibi.
"The son said he was home but never saw (Yang) in or about the house," the Chief said.
Police learned that the relationship between Marni Yang and her oldest son was strained. He didn't back his mother's story, and there was something he said to investigators that struck them even more.
"He told the investigators that after hearing and seeing the story on TV -- some of his friends brought it to their attention -- that one of his gut reactions is that she somehow might be involved," Sliozis said. He said the son's unusual response left every investigator thinking, "That's an honest response. The son would have no other reason to answer that way."
Police say Yang also said her children were the reason she bought two books on how to make a silencer -- a silencer authorities accuse Yang of using during the shooting.
Yang told police that she bought the books for a school science project for one of her kids.
"We didn't ask the kids. But the investigators knew that the books were purchased during the summer. School was not in session. So what she was saying just didn't make sense," Sliozis said.
Yang later changed her story, police said, saying that she actually bought the books as a gift for an acquaintance of hers.
Lake County State's Attorney Michael Waller said he wouldn't want to call the Yang children to testify in their mother's murder trial unless it was absolutely necessary.
"If it's not necessary, why put children through this?" the State's Attorney said. "I mean, they're -- they've been -- in a sense, been victimized by their mother's terrible crime. You know, they've been subject to police questioning. And now, I can imagine what has happened to their lives. And if we need to use 'em as witnesses to prove our case, we will. If we don't need to, we won't."
Prosecutors say Yang's own words may be far more incriminating than anything her children said. Police say they caught her on tape confessing to facts only the killer could know.