A man accused of stabbing his girlfriend to death with a steak knife was blind with rage at the time of the crime and didn't know what he was doing, his attorney told jurors Friday.

Armando Lopez is on trial in Pima County Superior Court for first-degree murder in the killing of killing Marie Mesa, 44, in January 2012.

"He was in a rage when he stabbed her," defense attorney John Sando told jurors Friday.

Sando argued throughout the trial that Lopez lost control the night he beat and stabbed Mesa because he believed she had been cheating on him.

In testimony, Lopez also said he had blacked out and couldn't remember what happened.

"He was obviously in a serious emotional state, and because of that emotional state, or passion, which is why we have manslaughter," Sando said.

Deputy County Attorney Michelle Araneta disputed the claim, telling jurors Lopez's memory for details of the events leading up to the crime was fine.

"He wants you to believe he had a blackout because he doesn't want you to hold him accountable for first-degree murder," Araneta told jurors.

She argued Lopez committed premeditated murder because he had ample "cooling off" periods before the dispute escalated into violence.

On the night Mesa was killed, she and Lopez had been at a bar with friends. When they returned to Mesa's northwest-side house, they began to argue over a letter that Lopez thought provided proof that Mesa was having an affair.

Araneta said Lopez found the letter while Mesa was in the bathroom but waited for her to come out before confronting her.

"There was not a sudden quarrel," Araneta said.

Mesa's teenage daughter, Tayler Ybarra, who was 15 at the time, called 911 to report the fight. Ybarra testified at the trial, and the 911 recording also was played.

"My mom's boyfriend is hitting her and she's bleeding," Ybarra tells the emergency operator in the recording.

Pima County sheriff's Deputy Sean Stoermer testified that Lopez had blood on his clothes and parts of his body but was still responsive when the deputy arrived at the house.

"He responded, 'No, she'd been cheating on me,' and that's all he said," Stoermer said of his questioning of Lopez.

Investigation documents say Lopez admitted to police that he killed Mesa.

Sando, however, pointed out police reports describing Lopez as "dazed" and "lightheaded" and noted he had urinated on himself.

The doctor who performed the postmortem examination testified Mesa had 15 stab wounds to her upper body, arms and legs.

One of the wounds cut into Mesa's aorta. Her body also had signs of asphyxiation.

Lopez wept and covered his face when photos of Mesa's body were placed on the overhead projector for jurors to see.

Sando argued the excessive number of stab wounds was proof of Lopez's uncontrollable state of mind.

"At that point he lost the ability to make conscience decisions," Sando said. "This shows rage."

The jury began deliberations Friday and plans to resume Monday.

Contact reporter Patrick McNamara at 573-4241 or pmcnamara@azstarnet.com