Investigators found two blood-stained knives; a steak knife in the kitchen sink and another in Plackowska's car, Berlin said.
Still covered in blood and with scratches on her hands, Plackowska drove to a Catholic church. Finding it closed, she called the church and left a message saying she had "done something bad" and needed help, Berlin said. She then went to a friend's home where her other, adult son was staying and said she had been robbed.
About the same time, Olivia's mother, Marta Dworakowski, returned home to discover her door locked and the babysitter's car gone. She called police to report her daughter was missing, and officers forced their way into the home.
Police took Plackowska into custody at her friend's home, and prosecutors charged her with first-degree murder late Wednesday.
Plackowska initially told investigators that an intruder had broken into the home and killed the children while she was outside smoking a cigarette.
"She then changed her story and stated that she was really battling the devil ... and that the children had evil inside them and she was trying to get the evil and devil out of them," Berlin said.
Later, she admitted she had lashed out in anger at her husband because he often left her alone to care for Justin and she worked as a cleaner, which she felt was beneath her, Berlin said.
In a brief telephone interview, Plackowska's husband, Artur, denied the two were having problems.
"The day before (the killings) she told me that she loves me," he said.
However, a neighbor said he frequently heard the couple shouting at each other over the last five years.
"It happened once every other month," Victor Tuckenberry said.
There also were money troubles. Public records show the couple filed a Chapter 7 bankruptcy in 2009; a foreclosure in her name only was filed in 2007.
Still, Tuckenberry said Plackowska doted on her sons and he was shocked by the slayings. Plackowska and Justin "were together all the time," he said.
Plackowska arrived in the United States from her native Poland on a tourist visa 12 years ago, Berlin said. She is not a U.S. citizen and authorities were trying to verify her immigration status. Berlin said she has no prior history of violence, although she had a misdemeanor DUI from about 10 years ago.
Plackowska didn't speak during Thursday's bond hearing other than to indicate she could not afford an attorney. The judge appointed a public defender.
Babwin reported from Chicago. AP News Researcher Jennifer Farrar contributed to this report from New York.