Story highlights The 14-year-old called 911, police said

Officers found her sister upstairs stabbed 40 times

Neighbors say the sisters seemed close and played together

The 14-year-old is in a juvenile detention center

An 11-year-old Illinois girl was stabbed 40 times by her 14-year-old sister, police say, and the reason why has many in the small Chicago-area community of Mundelein reeling in disbelief.

Police found the 11-year-old girl unconscious and in need of medical attention in an upstairs bedroom of the dwelling Tuesday morning. She later died.

Because the sister, who called police, initially told officers an intruder was to blame for the attack, authorities briefly placed three schools on lockdown.

The truth, when it emerged, was even more shocking.

The 14-year-old, authorities say, repeatedly and fatally stabbed the victim -- her half sister -- with a kitchen knife because the teen thought she was ungrateful.

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"This incident is a heartbreaking tragedy that defies understanding," said Mundelein Mayor Steve Lentz. "I am asking the Chicagoland area, please pray for us. Pray, first of all, for the family that has been devastated by this."

A group called "Mundelein Moms" is raising money for the girl's funeral and said it raised $6,381 in just one day. There also is an effort to have residents leave their lights on Friday night in remembrance.

Intruder in the house

Police received a 911 call about the incident at 8:30 a.m Tuesday. The call came from the 14-year-old girl, said Mundelein Police Chief Eric Guenther.

The teen told emergency dispatchers that an intruder had barged into the home, attacked her sister and then fled, CNN affiliate WLS reported

Schools in area were immediately placed on lockdown, Guenther said.

"The area-wide school lockdown lasted only 15 minutes," he said.

But the teen's story unraveled quickly. Under police questioning, she admitted to the attack, authorities said.

Feeling unappreciated

The teen told police she was mad that her sister didn't appreciate all she did for her, prosecutors said at the suspect's detention hearing Wednesday. WLS was at the hearing.

The teen said she cooked dinner for her younger sibling and performed other chores. She was also angry that the 11-year-old girl had recently hit her.

The night before the attack, the teen thought about it for about 10 or 15 minutes, then went downstairs and grabbed a knife, according to prosecutors.

After the stabbing, she took a shower to wash off the blood, called police and made up the story about the intruder, prosecutors said.

The 14-year-old was arrested and was charged with murder.

The case is in juvenile court and a decision could be made in the future to move the case to adult court, the Lake County State Attorney's Office said.

Playing together

Those who knew the girls said they lived at the home with their mother. Neighbors said the siblings seemed close.

The girls were often seen playing together and practicing cheerleading routines, WLS reported.

"They would always be together," Mary Ann Gryder, a neighbor told the affiliate.

"The older one would be taking care of the younger one, and vice versa."