The Houston Chronicle reported that Thomas suffers from schizophrenia, characterized by psychotic delusions and "hyper-religious preoccupations." His lawyers have argued he was "actively psychotic" when he committed the crimes for which he was convicted.
Thomas was found guilty of the March 27, 2004, slayings of Laura Christine Boren, 20, the couple's son, Andre Lee Boren, 4, and Boren's infant daughter, Leyha Marie Hughes. According to KSAT, the three victims were stabbed to death and their hearts ripped from their chests.
In Boren's case, Thomas missed her heart and removed a portion of her lung instead, according to a 2013 profile of Thomas in Texas Monthly. Thomas, then 21 years old, stabbed himself in the chest multiple times, laid on the floor next to his wife's body and waited to die, the magazine said.
When he didn’t, he pocketed the organs he had cut out and walked five miles to his home. He went to the Sherman Police Department a few hours later and confessed, Texas Monthly reported.
"I thought it was what God wanted me to do," Thomas told investigators, according to the magazine.
After he plucked out his eye while awaiting trial in the Grayson County Jail, Thomas repeatedly asked hospital personnel to let him see his wife and beg her forgiveness, which he said he had already obtained from the children.
"I love her, and I need her to forgive me," he told them of Boren, Texas Monthly said.
Court documents filed before Tuesday's oral arguments said Thomas' mental illness went undiagnosed and untreated "until after the delusions it caused led him to commit the crime here," KSAT reported. One of his attorneys, Catherine M.A. Carroll, also argued that her client's trial lawyers failed to challenge a judge's ruling that found him competent to stand trial.
Carroll also claimed Thomas' trial lawyers did not request a competency hearing and that jurors were not given enough information on the defendant's long history of mental illness, the news station said.
Texas Monthly reported Thomas began telling classmates about the voices in his head when he was as young as 10 years old. By the age of 13, Thomas had made his second suicide attempt, the magazine reported.
Carroll also alleged that the jury in the case was racially biased against Thomas, who is black. Boren, his estranged wife, was white.
The Chronicle reported that the jury that convicted Thomas was all white. Four of the jurors were allowed to serve, despite expressing opposition to interracial marriage.
One juror said it was not "what God intended," and another said white people "should stay with (their) bloodline," the newspaper reported.
Prosecutors are contesting the appeal, according to KSAT. They have argued that, while mentally disabled people are barred from being executed, the courts have not extended that same prohibition to the mentally ill.
"The Fifth Circuit has consistently refused to find a connection between the intellectually disabled and the mentally ill, repeatedly rejecting arguments like the one Thomas makes now," Assistant Texas Attorney General Fredericka Sargent wrote, according to the news station.
A federal judge in 2016 rejected Thomas’ appeal, prompting this week’s oral arguments seeking the ability to keep the appeal going.