Warning: This story includes graphic allegations of violence against children

EXCLUSIVE: A suspected MS-13 gang member set to go on trial next week for the horrific 2018 slaying of his California girlfriend's 10-year-old son is a native of El Salvador and living in the U.S. illegally, according to a law enforcement source.

Kareem Ernesto Leiva, 37, and his American girlfriend Heather Maxine Barron, 33, are both charged with murder and torture in the death of her son, Anthony Avalos, as well as child abuse against two other children in the home, court records show.

Prosecutors allege the boy had been beaten, starved, forced to kneel on rice, force-fed, whipped and more in a home where he was allegedly subjected to "extreme physical pain and suffering." 

CALIFORNIA SEEKS DEATH PENALTY IN ALLEGED TORTURE KILLING OF ANTHONY AVALOS

Leiva in prison garb on left, Avalos in T-shirt on right

Kareem Leiva, an illegal immigrant from El Salvador, is accused of the torture and murder of 10-year-old California boy Anthony Avalos, right. (Al Seib/Los Angeles Times via Getty Images, David Barron via AP)

While in jail, Leiva allegedly shanked another inmate, according to court documents, and was accused of domestic violence against females in both 2010 and 2013.

His brother, Mauricio Leiva, is another alleged MS-13 member who was indicted in a federal racketeering case against a deadly drug ring in 2016, court documents show. 

Leiva and Barron's murder trial in Los Angeles is scheduled to begin next week. Jonathan Hatami, who rose to prominence as the lead prosecutor in the Gabriel Fernandez trial, and Saeed Teymouri are assigned to the case. 

Leiva's defense attorney, Dan Chambers, did not immediately respond to a message seeking comment.

 Anthony Avalos was smiles in old portrait

This undated photo provided by David Barron shows Anthony Avalos. Los Angeles County prosecutors have charged his mother and her boyfriend with murder and torture. (David Barron via AP)

Prosecutors previously alleged that Barron and Leiva whipped the child, poured hot sauce on his face and hung him upside down. He had been tortured "for almost two weeks, up until paramedics responded to [the] residence and found Anthony's lifeless body," court documents allege.

Anthony and his siblings also suffered other forms of abuse, including being burned with a curling iron or locked in their rooms for hours.

On June 20, 2018, just weeks after the end of Anthony's fourth-grade school year, Barron called 911 to report her son was unconscious, according to the documents. 

Police arrived and found the boy not breathing and covered in bruises and abrasions. He also had circular burn marks on his stomach. After he was transported to a hospital, doctors noted he appeared "severely malnourished and dehydrated." 

Heather Maxine Barron in jail jumpsuit and shackles

Heather Maxine Barron, who was indicted for the torture and murder of her 10-year-old son Anthony Avalos in Lancaster, walks into court for a pretrial hearing Feb. 27, 2018, in Los Angeles Criminal Court. (Irfan Khan/Los Angeles Times via Getty Images)

MOTHER'S BOYFRIEND ARRESTED IN DEATH OF 10-YEAR-OLD BOY WHO COMPLAINED ABOUT ABUSE

He was pronounced dead the following morning.

The Los Angeles County Medical Examiner-Coroner ruled Anthony's death a homicide and identified multiple causes of death: subdural, subarachnoid and intraparenchymal cerebral hemorrhages due to blunt-force head trauma.

 Anthony avalos in brown shirt

Anthony Avalos in an undated family photo. (Facebook)

Court documents allege he was held up by his feet and dropped on his head repeatedly, punished with wrestling moves, forced to fight other children in the house, thrown into furniture, beaten in the face with a ping-pong paddle, slammed to the floor and given rug burn.

The day before the 911 call, Anthony was unable to walk or eat, according to authorities. But Leiva and Barron allegedly left him unconscious for hours.

Kareem Ernesto Leiva in jail jumpsuit sitting in court

Kareem Ernesto Leiva at a pre-trial hearing in Los Angeles Criminal Court Feb. 27, 2018, in Los Angeles. Leiva and his girlfriend Heather Maxine Barron were indicted for the murder and torture of her 10-year-old son Anthony Avalos in Lancaster. (Irfan Khan/Los Angeles Times via Getty Images)

"The final death blows came that evening when Leiva slammed Anthony on his head multiple times," prosecutors allege. "Barron did not call the police until the next morning. Leiva purposefully fled the residence with his children before authorities arrived."

Authorities had received reports for years of alleged child abuse, according to court filings. Even before Barron began dating Leiva, another adult male acquaintance of hers was accused of sexually assaulting her son when he was just 5 years old.

Despite many interactions with county child services, authorities determined investigations into numerous prior child abuse allegations were "inconclusive." 

Kareem Leiva, left, Heather Barron, right

Booking photos showing accused child killers Kareem Leiva and Heather Barron. (Los Angeles Sheriff)

Barron was able to keep custody of her seven children, three of whom she shared with Leiva, who fathered another five kids with three other women. According to authorities, he was especially abusive to his non-biological children, and allegedly admitted to shoving Anthony's younger brother so hard into a chair he needed three staples to close a gash in his scalp.

FOX Los Angeles reported in October that the Los Angeles County Board of Supervisors approved a $32 million settlement with the rest of Anthony's family, which sued, alleging social workers on the case failed to properly handle the accusations. Members include the boy's father as well as maternal aunts and uncles who are expected to testify against Barron at trial.

Barron and Leiva both appear in court behind glass

Heather Barron appeared for a hearing in a Lancaster courtroom with boyfriend Kareem Ernesto Leiva, both charged with the torture and murder of Barron's 10-year-old son, Anthony Avalos, Aug. 3, 2018. (Al Seib/Los Angeles Times via Getty Images)

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The former Los Angeles district attorney, Jackie Lacey, had intended to seek the death penalty in the case, but current DA George Gascon, who has publicly opposed such punishment for years, reversed course after taking office.