HOUSTON -- A convicted killer's mother took the stand in the punishment phase of her son's trial and begged the jury to spare his life, KPRC Local 2 reported.

Dexter Johnson, 19, was found guilty of the capital murder of Maria Sally Aparece, 23, on June 13

Her boyfriend, Huy Ngo, 17, was also killed in the crime spree last summer.

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Johnson's mother told the jury that her son's father was never a part of his life. She said her boyfriends verbally and physically abused her son and two daughters.

She told the jury that she was very poor when raising her children and once tried to give them away.

Johnson's sister told the jury that her brother was a loner and was often teased.

Dr. Ruben Gur testified that Johnson underwent an MRI and PET scan in May and June that indicated he has significant brain abnormalities.

Defense attorneys said Johnson fell on his head when he was 6 months old. Gur testified that Johnson may have brain damage from the fall.

Attorneys said Johnson started hearing voices when he was 15 years old, demonstrating characteristics of schizophrenia.

"We're trying to put together, or piece together for the jury, his entire life so they can understand the difficulties he encountered as a child as a result of the problems that he had," defense attorney Anthony Osso said.

Johnson began failing in the second grade and had a rough childhood filled with neglect, attorneys said.

Aparece and Ngo's bodies were found in an overgrown field in the 10400 block of Gateway Drive on June 23, five days after they disappeared.

Prosecutors said Johnson robbed, kidnapped, shot and raped Aparece. Johnson's attorneys claimed he was not the shooter.

Timothy Randle, 19, Alvie Butler, 25, Keithron Fields, 27, and Ashley Ervin, 17, were also arrested and charged with capital murder in connection with the killings.

Johnson, Fields and Ervin were also charged in connection with Brady Davis' death on May 26, 2006. Davis was robbed, shot and killed at a northeast Houston car wash.