"Ladies and gentlemen, on behalf of the participants in this trial, I wish to thank you for your extraordinary service to this community," she told them. "This was not your typical trial. You were asked to perform very difficult responsibilities."
An alternate juror and a juror who was on the panel cried as the verdict was being read.
As the jury filed out of the courtroom, one juror said, "I'm sorry" to Alexander's family.
Jurors struggle to agree
Jurors had deliberated for more than 13 hours in the penalty phase of the trial when they told the court they wouldn't be able to agree on a verdict.
Earlier this month, the same jurors took less than two hours to decide that Arias was "exceptionally cruel" when she stabbed Travis Alexander 29 times, slit his neck from ear to ear and shot him in the face.
They pronounced her guilty of first-degree murder on May 8 after 15 hours of deliberations.
For Arias to be sentenced to death, a jury's decision must be unanimous.
About an hour into its deliberations on Thursday, the jury sent out a question, but the details of its query were not revealed in court.
On Wednesday morning, the jury sent out a note saying its members couldn't agree. Stephens told them to try again and ordered them back into the jury room.
If Arias is given a sentence of death, she would be the fourth woman on death row in the state of Arizona.
A plea for mercy
On Tuesday, Arias pleaded with jurors to spare her.
She told them she would dedicate her time in prison to performing acts of charity from behind bars. She said she would teach people to read or to speak Spanish, start a book club and donate her hair so it could be used to make wigs for sick children. She showed jurors several pieces of her artwork.
She called Alexander's murder "the worst mistake" she had ever made, "the worst thing I've ever done." She couldn't have imagined herself capable of such a grisly crime, Arias told the jury.
"But I know that I was," she said. "And for that I'm going to be sorry for the rest of my life -- probably longer."
A case packed with twists and turns
Arias was living in Yreka, California, when she met Alexander at a business convention in Las Vegas in September 2006. That November, he baptized Arias into the Mormon faith, a ceremony Arias said was followed by anal sex.
Arias became his girlfriend two months later, she testified. They broke up in the summer of 2007, and Alexander began dating other women.
In June 2008, Alexander missed two appointments, prompting friends to go to his house. They found his naked body crammed in a stand-up shower
He had been stabbed 29 times in the back and torso and shot in the head. His throat was slit.
After her arrest, Arias told an elaborate lie about masked intruders breaking into Alexander's house and killing him before she narrowly escaped.
Relatives who spoke with police described her as mentally unstable.
The trial, which started in January, has been rife with sex, lies and digital images -- among them graphic autopsy photos that showed Alexander's body.
Now, if the trial continues, some of the same gruesome details of his slaying may be presented again in court.