Meet Cristian Fernandez: 13-year-old boy who is being tried as an adult for killing his 2-year-old brother!

Police and family services records have given a startling insight into the heartbreaking childhood of a 13-year-old boy accused of beating his two-year-old brother to death.

Cristian Fernandez reportedly suffered neglect at the hands of a teenage mother and a drug-abusing grandmother, sexual abuse by a cousin and physical assaults by a stepfather who went on to kill himself.

Fernandez, who was just 12 when he fatally smashed his brother's head against a book shelf, could become America's youngest ever 'lifer' after being charged as an adult over the murder.

But the case has ignited a fierce debate over whether the punishment is too strict for a child - particularly for one who was left to languish in a poisonous upbringing littered with abuse.

Fernandez was born in Miami, Florida in 1999 to Biannela Susana, who was just 12. The boy's 25-year-old father received 10 years' probation for sexually assaulting her.

Two years later, both mother and son went to foster care after authorities found the toddler filthy, naked and walking in the street at 4am near the motel where his grandmother, who was taking care of him, was found surrounded by cocaine.

In 2007, when Fernandez was eight, the Department of Children and Families investigated a report that he was sexually molested by an older cousin. Officials noted there were other troubling incidents: he killed a kitten; he simulated sex with classmates; he masturbated at school.

By October 2010, Fernandez and his mother were living with her new husband when he suffered an eye injury so bad that his school sent him to the hospital where he was examined for retinal damage.

Fernandez told officers that his stepfather had punched him. When officers went to the family's apartment, they found the stepfather dead from a self-inflicted gunshot wound.

The family moved north to Jacksonville and Fernandez enrolled in middle school, getting straight A's, and they settled in a bland, beige public housing complex.

On June 3, 2011, deputies were called to the apartment: Fernandez's baby brother, David, was dead inside. He was found to have a fractured skull, bruising to his left eye and a bleeding brain.

Susana, then 25, admitted to investigators that she had left Fernandez, David and her other children home alone. When she returned, she said she found David unconscious.

It later emerged that Susana surfed the internet for four hours while her youngest child lay dying, and she even admitted to looking for information on child concussions before eventually calling police.

When they arrived, Susana revealed that two weeks before David's death, Cristian had broken the toddler's leg while wrestling.

The medical examiner said David might have survived if she had taken him to the hospital sooner for the head injury and she was charged with aggravated manslaughter. She pleaded guilty in March and could get 30 years behind bars.

Fernandez was charged with first-degree murder. Another felony charge was filed after his five-year-old half-brother told a psychiatrist that Fernandez had sexually assaulted him.

The boy has talked openly to investigators and therapists about his crimes and his life; the gritty details are captured in various court documents.

'Cristian denied any plans or intent to kill his brother,' one doctor wrote. 'He seemed rather defensive about discussing what triggered his anger.

'He talked about having a "flashback" of the abuse by his stepfather as the motive for this offense... Cristian was rather detached emotionally while discussing the incident.'

Fernandez has been charged as an adult and is the youngest inmate awaiting trial in Duval County.

If convicted of either crime, Fernandez could face a life sentence - a possibility that has stirred strong emotions among those for and against such strict punishment.

The case is one of the most complex and difficult in Florida's courts, and it could change how first-degree murder charges involving juvenile defendants are handled statewide.

Cristian's mother Biannela Susana