Story highlights Mom repeatedly stabs son, 5, friend's 7-year-old girl, 2 dogs, police say

Her motive was to cause her husband pain, police say

Police say they "walked into a scene of unimaginable horror"

A Chicago-area woman has been charged with killing her 5-year-old son and a 7-year-old girl as she babysat them, allegedly stabbing each dozens of times in a bedroom this week as they pleaded for their lives, authorities say.

Elzbieta Plackowska, 40, of Naperville, Illinois, allegedly told police she instructed her son Justin Plackowska and the girl, Olivia Dworakowski, to kneel and pray before she fatally stabbed them and two dogs at the girl's home on Tuesday evening, Naperville police said.

Plackowska -- charged Thursday with two counts of first-degree murder -- allegedly gave investigators a number of reasons why she killed the children, the most recent of which was that she was upset with her husband and wanted to cause him pain, according to police.

"She began stabbing her son Justin and told him he was going to heaven tonight," DuPage County prosecutor Bob Berlin told reporters Thursday, citing Plackowska's alleged statements to police. "He pleaded for his life and told her to stop, but she continued stabbing him until he was dead."

Plackowska was being held Thursday without opportunity for bail. If convicted, she would face a mandatory sentence of life in prison with no possibility of parole.

CNN's attempts to reach lawyer Mike Mara, a DuPage County public defender representing Plackowska, weren't immediately successful.

Police officers found the children's bodies in Olivia's home Tuesday night after Elzbieta Plackowska, covered in blood, arrived at a nearby friend's house and claimed she had been robbed, police said.

The friend called police. Around the same time, Olivia's mother called police to say she had come home from work and couldn't enter the house because there was a strange lock on the door, and that she couldn't find Plackowska or the children, Berlin said.

Officers forced entry into the home and "walked into a scene of unimaginable horror," finding the bodies on the floor of the master bedroom, Berlin said.

Investigators said Plackowska offered several explanations, including that she believed an intruder killed the children, police said. Later, she told police that the children had evil inside them and she needed to drive the devil out, authorities said.

"She ultimately admitted that she had lied to the police in her earlier statements and stated she was angry with her husband," Berlin said.

Plackowska told investigators that she told the children to get ready for bed, and the kids entered the master bedroom, Berlin said. Plackowska said she then went to the kitchen, took two knives, entered the bedroom and told the children to kneel on the floor and pray, according to Berlin.

She told police she stabbed Justin repeatedly, and then stabbed Olivia because Olivia saw her attacking Justin, according to Berlin. She said Olivia, like Justin, pleaded for her life, Berlin said.

Police said Justin was stabbed about 100 times, and Olivia about 50 times.

"She did not express any remorse," Berlin said.

Naperville Police Chief Bob Marshall told reporters that the "senseless taking of the lives of these two children defies logic, and our community is grieving."

In all my years of law enforcement, this is the most gruesome and horrific crime scene that I have seen," Marshall said at the news conference.

Plackowska told police she was upset with her husband in part because he was gone most days, working as a truck driver, and that she felt he didn't treat her as she deserved.

Plackowska is awaiting a November 21 arraignment.