UPDATE: 11/1/2012 – 4:04 p.m.

NAPERVILLE (CBS) – Before killing her own 7-year-old son and the 5-year-old girl she was babysitting, Elzbieta Plackowska allegedly told them to get on their knees and pray.

Prosecutors said Plackowska admitted stabbing her son, Justin Plackowski, more than 100 times because she wanted to hurt her husband, but only after first blaming the brutal murders on a robber, then saying she heard the devil’s voice and killed the children to save them from demons.

CBS 2’s Suzanne Le Mignot reports authorities called the scene where they found Justin’s body alongside that of 5-year-old Olivia Dworakowski one of the most disturbing they’d ever seen.

Plackowska, 40, was charged with two counts of first-degree murder late Wednesday night. She was ordered held without bail Thursday morning.

When police arrived at the scene in the 800 block of Quinn Court, they “walked into a scene of unimaginable horror,” DuPage County State’s Attorney Bob Berlin said at a news conference on Thursday.

LISTEN: WBBM Newsradio’s Bernie Tafoya Reports

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Naperville Police Chief Bob Marshall called it the most gruesome and horrific crime scene he had ever witnessed in over 30 years of law enforcement.

Prosecutors said, before killing Justin and Olivia Tuesday night, Plackowska took them to St. Elizabeth Seton Catholic Church in Naperville. A church bulletin indicates she and her husband have been parishioners since 2011. Upon arriving at the church, she couldn’t get in. She also went back to the church after the murders.

“She stated that she called the church, no one answered, and she left a message,” Berlin said. “She said she had done something bad and had needed assistance.”

Olivia’s mother called police late Tuesday night, because she couldn’t get into her home. Police broke down the door and officers walked down a blood-soaked hallway to the master bedroom.

There, they found Justin’s and Olivia’s bodies. Justin had been stabbed 100 times. Olivia had been stabbed 50 times.

“She made both victims kneel and begin praying. She began stabbing her son, Justin, and told him he was going to heaven tonight. He pleaded for his life and told her to stop,” Berlin said. “But she continued stabbing him until he was dead. She then turned her attention to 5-year-old Olivia, and began stabbing her with the knife multiple times. She, as well, pleaded for her life.”

Two dogs were also found killed inside the home.

Plackowska was found at the house of a friend who had called police.

After she was arrested, Plackowska first claimed an intruder killed the children and two family dogs while she was outside smoking a cigarette.

“She then changed her story, and stated that she was really battling the devil inside of the Quinn court address, and that the children had evil inside them, and she was trying to get the evil and devil out of them,” Berlin said. “She changed her story again, and stated that she, in fact, had stabbed both victims because they were poisoned by society, and did not listen like they should have.”

After she was taken to the Naperville police station, she finally admitted those were lies, and said she stabbed the children to death to get back at her husband.

“She stated that she was angry with her husband, because he was gone most days as an over-the-road trucker. She was left to care for Justin in the house, and she also had to work as a cleaning lady, which she felt was beneath her,” Berlin said.

He also said Plackowska claimed her husband didn’t treat her the way she deserved, and didn’t appreciate how good of a wife she was.

“She told the detectives that she thought, by killing Justin, she would make her husband hurt the way she hurt in their relationship,” Berlin said.

Plackowska allegedly said she killed Olivia because she had witnessed Justin’s murder.

Plackowska’s husband, Artur Plackowski, told the Associated Press he has no idea why his wife killed Justin and Olivia. He said she had told him she loved him the day before the murders, and he denied the two were having problems.

Police recovered two knives believed to have been used in the killings; one in the kitchen of the home, the other in Plackowska’s car.

Naperville police said Plackowska had been behaving erratically just before the two stabbing deaths.

The mother of the slain girl, Marta Dworakowski, called police after she arrived home from work on Thursday night. She became concerned because a door that was normally opened was locked.

Plackowska’s son attended Scott Elementary school. Olivia attended Brookdale Elementary School. Grief counselors

Counselors have been at both schools to help both the students and teachers in the aftermath of this tragedy.

Chief Marshall said police got to know Olivia after a mix-up during the first day of kindergarten this year.

Police were called to search for the girl after she didn’t turn up in class. It turns out Olivia had mistakenly gone to the wrong classroom. Marshall said officers had lunch with her that day.

The police and fire personnel who responded to the murder scene also will undergo counseling.

Plackowska was due to return to court on Nov. 21 for a preliminary hearing.