Updated

A convicted sex offender who killed 11 women was an upstanding person until a heart attack that went untreated for weeks changed his life, his lawyer said Monday. A defense expert testified that it could have even caused brain damage.

Anthony Sowell's defense attorneys, who didn't put a witness on the stand during his trial, began presenting evidence Monday as they try to spare him from being executed.

But a prosecutor told the same jurors who convicted Sowell, 51, that the defendant's life circumstances don't outweigh his crimes and that he should face the death penalty.

Sowell, dressed in a yellow dress shirt and dark slacks, slumped back in his chair and mostly stared straight ahead as defense attorney John Parker and assistant Prosecutor Richard Bombik presented differing images of him.

The jury, which must decide whether to recommend the death penalty or life in prison without parole, heard for the first time Monday that Sowell spent 15 years in prison for a 1989 attempted rape.

His criminal background wasn't allowed to be discussed during the trial to prevent prejudicing jurors. Sowell rested his chin on his right hand and several jurors glanced his way as Parker mentioned his earlier crime.

Jurors also heard from Parker that Sowell is expected to take the stand, likely an appeal to spare his life. The judge will allow Sowell's attorneys to coach his statement with leading questions, but prosecutors will not have a chance to cross-examine him.

The first defense witness, neuropsychologist Dale Watson of Livermore, Calif., said Sowell has a brain problem that could have been caused by a heart attack.

Watson, who has extensive experience testifying on how damage to the brain can affect behavior, said Sowell's IQ, mental-processing speed and handling of spelling, puzzle and drawing tests rank him low compared to other people.

A heart attack can affect processing speed by reducing oxygen to the brain, Watson testified.

Under cross-examination, Watson acknowledged that Sowell offered conflicting statements on whether he heard voices. He also said he doubted Sowell's subsequent denial that he heard voices.

Bombik also pressed Watson about Sowell's alcohol and drug abuse and whether that could have affected his brain.

"I can't be certain," Watson responded.

The prosecution had a chance to offer evidence to back its push for the death penalty, but limited its case to introducing its trial exhibits, which included graphic autopsy and crime-scene photos which the judge called "grotesque."

Bombik thanked the jury for reaching "the appropriate verdict" and said the panel still had a "very tough decision" to make.

He said prosecutors believed Sowell's pattern of crime merited the death sentence and said the state would challenge any defense testimony calling into question Sowell's mental state.

Parker, in his opening statement to the jury, portrayed Sowell as a person who had been raised in a violent, abusive home and joined the Marines after high school to create a new life for himself.

Parker said Sowell had been a hard-working person but suffered a heart attack in 2007 while shoveling snow and didn't get medical treatment for two or three weeks. His medical ordeal left him unable to work in 2007, about the time, Parker pointed out, that the victims began disappearing.

"I'm not suggesting he's a perfect person by any means," Parker said.

Investigators said Sowell lured victims to his home with the promise of alcohol or drugs. Police discovered the first two bodies and a freshly dug grave in late 2009 after officers went to investigate a woman's report that she had been raped there.

Many of the women found in Sowell's home had been missing for weeks or months, and some had criminal records. They were disposed of in garbage bags and plastic sheets, then dumped in various parts of the house and yard. All that remained of one victim, Leshanda Long, was her skull, which was found in a bucket in the basement.