SAN ANTONIO -- The son of slain Alamo Heights teacher Diane Tilly testified Tuesday in the sentencing phase of the capital murder trial of the man convicted of killing the educator.

Stephan Tilly, who lives in Austin, told jurors how the death of the beloved teacher has affected his family, especially the victim's daughter.

"She can't sleep at night," Tilly said on the witness stand. "She locks the doors. She checks the doors every time she's in the house. She won't let her sons out of her sight."

Stephan Tilly was the last prosecution witness to testify in the punishment phase of the trial of Ronnie James Neal, who was found guilty of capital murder in March of killing Diane Tilly in November 2004.

Following Stephan Tilly's testimony, defense lawyers began trying to convince jurors that Neal is mentally retarded, and therefore should not be executed by lethal injection.

Joann Murphy, a clinical psychologist, outlined three definitions of mental retardation that are recognized by the state of Texas, including the American Psychological Association, the American Association For Mental Retardation and the State Health and Safety Code.

Defense lawyers claim that Neal is mentally retarded, based on an evaluations when he was a teenager.

Prosecutors pointed out that a current IQ test has not been administered to Neal and that a psychologist has not visited with the defendant, who refuses to attend the punishment phase of his trial, saying in a letter he wrote to the judge that "it don't matter."