PHOENIX (3TV/CBS 5) - It's an unimaginable act: a mother killing her children. But Phoenix police say 22-year-old Rachel Henry admitted to harming her three kids Monday and was booked into Fourth Avenue Jail on suspicion of first-degree murder.

[WATCH: Psychologist weighs in on Phoenix mother who admitted to killing her 3 kids]

"It's mind-blowing, it's shocking. It's one of those things where you start asking yourself, why didn't she reach out for help?" said psychologist Dr. John Delatorre.

[RELATED: Police ID mom who admitted killing her three children under age 4 at Phoenix home]

And while it's horrified the state, Delatorre says the kind of crime Henry is accused of is more common than many might think.

"The most recent statistics that I've been able to find has this occurring about 3% of all homicides are filicides, which is the parents murdering their child," Delatorre said.

Delatorre says parents often kill their children because they feel they have no other option. Tuesday, Delatorre offered up different areas investigators might look into.

[WATCH: Mom booked on suspicion of first-degree murder]

"Could it be mental illness? Absolutely it could be mental illness. Could it be that the person saw no way out? That here they have to take care of so many children and they don't have enough resources to care for them."

[VIDEO: Family member of 3 kids killed by mother in Phoenix shares heartbreaking images, video]

And in this case, Phoenix police say investigators will be looking into whether postpartum depression might have been a factor. The 22-year-old mother had three children, all under four-years-old.

[PHOTOS: See the Phoenix house where mom admitted to killing 3 children]

"Having an increase of hormones when she's pregnant, then a depletion after birth, and then so quickly another one, and then so quickly another one for someone who's so young could play such a havoc on the person's emotional experience because of the depletion of hormones," Delatorre said.

Delatorre added it would also be useful to know why Henry recently moved to Arizona from Oklahoma, and what her family life was like. He says with so little information, it's tough to tell why this happened.

"Why did it go this far? I think that's my initial reaction just as a human being," Delatorre said.


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