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." 

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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.

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." 

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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.

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.

"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." 

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.

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.