Prosecutor Juan Martinez came out swinging in his closing arguments in the Jodi Arias murder trial, calling Arias a liar and showing photos so graphic they reduced the sister of slain Travis Alexander to tears.

She's "attempted to manipulate you.”

That’s what the prosecutor told the jury Thursday, as closing arguments began in the most watched murder trial of the year: the Jodi Arias case. It’s been going on for 17 weeks, with a made-for-TV mix of kinky sex, horrifying violence, and, at the center of it all, a slight, soft-spoken defendant who admits she killed her ex-boyfriend, Travis Alexander.

The question for jurors is: Why?

Rob Schumacher / Pool via The Arizona Republic and AP Defendant Jodi Arias listens to prosecutor Juan Martinez make his closing arguments during her trial at Maricopa County Superior Court in Phoenix on Thursday, May 2. Arias is charged with first-degree murder in the stabbing and shooting death of Travis Alexander.

The prosecution, led by Juan Martinez, tells it this way: On June 4, 2008, Jodi Arias drove a thousand miles from Yreka, Calif., to Mesa, Ariz., where she showed up at Travis’ home. She’d rented a car, dyed her hair, turned off her cell phone—apparently to make her harder to identify, her movements harder to track. Her mission, prosecutors say: murder.

Arias and Alexander had broken up after a hot but secretive affair. Alexander, according to friends, was a successful salesman who liked the ladies. He was also a member of the Church of Latter Day Saints whose Mormon religion disapproves of premarital sex.

Travis’ friend Aaron Dewey, who knew them both well, told Dateline’s Josh Mankiewicz: “Jodi became Travis’ drug. He was able to get something from her that he couldn’t get anywhere else with the good Mormon girls that he typically dated.” Soon, Arias said on the stand, she was acting out Alexander’s every pornographic fantasy.

But while Arias may have been a hot girlfriend, she was not, apparently, Travis’ idea of a good Mormon wife. Friends said that even though she converted to his faith, he broke up with her and began dating—chastely, he told her—other women.

According to Alexander’s friends, who testified in the trial for the prosecution, Arias apparently didn’t take it well. Alexander’s friends say she stalked him and slashed his tires. The prosecutor argued that she eventually killed him.

The killing was shocking in its violence. After a day of sex, Arias admitted, she shot Travis in the face, stabbed him more than 20 times, and slit his throat from ear to ear.

At trial, Arias testified she killed Travis in self-defense. In her opening statement defense lawyer Jennifer Willmott told the jury, “Jodi had to make a choice. She would either live or she would die.”

Arias testified in her own defense—a marathon 18 days on the witness stand. She told the jury that Alexander demeaned and sometimes abused her throughout their relationship. She testified that the day she killed him started off with sex play—each photographed the other—but ended in violence when she dropped his camera—and, she claimed, he attacked her.

Her testimony: “He lunged at me and we fell…. And I got up and he's just screaming angry and after I broke away from him he said [I’ll] ‘f------ kill you, bitch.’”

She tearfully testified that she had no memory of stabbing Travis.

Arias’s sensational testimony made this case a TV and Internet event. A Twitter handle in her name, @JodiAnnArias, run by a friend had more than 35,000 followers at last count.

But only 12 people ultimately matter… The jurors who will decide her fate. Today, in closing arguments, prosecutor Juan Martinez told them: “This individual, the defendant, Jodi Ann Arias, killed Travis Alexander.... . Even after slashing his throat from ear-to-ear, none of you will convict her. Taking a gun, shooting him in the face. Absolutely none of it was her fault. It`s Travis Alexander`s misfortune. Everybody else is wrong."

“She scammed him. Are you going to allow her to scam you?.”

The defense is expected to present closing arguments on Friday. The jury will consider a wide range of possible verdicts, including first-degree murder, second-degree murder, manslaughter, and not guilty. If Arias is convicted of first-degree murder, the jury will also have to decide whether she deserves the death penalty.

Dateline has been covering the Jodi Arias trial from the start. After the verdict, watch for an hour-long report on the case, with new interviews and new information you won’t see anywhere else.

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