INDIANAPOLIS -- A 4-year-old boy fatally shot a 3-year-old girl at a home on Indianapolis' west side Thursday, leading police to arrest the girl's mother on child neglect and drug charges in what officers termed a very tragic incident.

Fiona Lee (pictured), 26, faces one count of felony child neglect, along with charges of possession of cocaine and possession of marijuana, after police were called to the home in the 200 block of North Sheffield Avenue just before 1 p.m.

Investigators found that the child, Aunesti Lee Allen, had been shot in the head.

Indianapolis police Lt. Jeff Duhamell said that Lee was upstairs and that four children were downstairs -- three of them hers, along with her boyfriend's 4-year-old son.

"She heard a shot, and she ran downstairs, and … the 4-year-old … had picked up the gun and pointed it at the 3-year-old, pulling the trigger," Duhamell said.

The gun had been left unattended on a kitchen counter, investigators said.

"(The gun) was easily accessible to the children," Duhamell said. "The mother had walked by it several times, however did not secure it."

Neighbors told police that Lee came out of the house yelling for help, saying that the girl had been shot.

"She was screaming. She was out of her mind crying," said neighbor Robert Tremblay. "She was losing it."

The girl was rushed to Riley Hospital for Children and was pronounced dead just after 4:30 p.m.

"These children had been known to play … with play guns," Duhamell said. "He probably thought it was a play gun and pulled the trigger."

According to a police report, officers found marijuana cigarette butts, along with a bag and a digital scale with cocaine residue, prompting the drug charges.

"It's heartbreaking. She's a little girl, and she had so much ahead of her," said neighbor Maria Carmona. "She was only 3 years old. It's a loss to the family. It's a loss to the neighborhood."

Police and Child Protective Services have yet to talk with the 4-year-old boy in an effort to determine intent.

"Very tragic situation -- we talk time and time again about having loaded guns in the house that are not put up," Duhamell said. "They have to be responsible. That gun should have never been in a position for those children to even get to it."

The other children, a 6-year-old girl and another 4-year-old boy, witnessed the shooting, Duhamell said.

"The lives of many people were, within a split second, turned upside down," Duhamell said.

Police aren't sure who the owner of the gun is.

"Yesterday's incident, very tragically, demonstrates that we have to have a larger, broader conversation about firearms in the home," said Marion County Public Safety Director Frank Straub. "There has to be an awareness to the fact that guns kill people."

Aunesti's cousin, Briana Golder, said she cared for the child often and is struggling to cope with her death.

"I think everybody's going to miss her, whether they met her for a little bit or knew her from the time she was born to when she died," Golder said.

Lee is on suicide watch at the Marion County Jail.