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Former Dallas police officer Amber Guyger's murder trial is underway in a Dallas, Texas courtroom.

Guyger, 31, is accused of fatally shooting Botham Jean after she entered his apartment under the mistaken belief that she was in her own apartment.

The former officer wore a modest blue dress on the first day of her murder trial on Monday.

After working a 13-hour shift, Guyger parked her car on the wrong floor and entered Jean's apartment through an unlocked door on Sept. 6, 2018.

According to her defense team, Jean, who was seated on his sofa eating a bowl of ice cream, was shot and killed when he refused her orders.

But prosecutors say Guyger was distracted by steamy text messages she exchanged with her lover, a fellow Dallas police officer.

The prosecutor argued that Guyger walked past 16 other apartments on the 4th floor and failed to notice she was not on the 3rd floor where her apartment was located directly under Jean's.

Dallas County Assistant District Attorney Jason Hermus said Guyger was also on the phone with her lover, Officer Martin Rivera, when she pulled into her apartment complex.

Hermus said Guyger sent a Snapchat message to Rivera at about 9:30 p.m. which read, "wanna touch," according to the Dallas Morning News. Earlier in the day, she texted Rivera that she was "super horny today," according to the prosecutor.

On the witness stand, Rivera testified he had a 16-minute phone conversation with Guyger as she drove home. He denied making plans with her for later that night.

Rivera, who was in a relationship with another woman, said he and Guyger were no longer lovers at the time.

Defense lawyer Robert Rogers said Guyger didn't notice the wrong number on the apartment door when she walked in and encountered Jean on his sofa in the darkened apartment.

"What was going through Amber's mind was just, 'I'm going home,'" Rogers said, according to the Morning News. "'I'm done with my day of work, I'm exhausted and I'm going home.'"

Rogers said Guyger, who is white, shot Jean, who is Black, in self-defense because she mistakenly believed she was in her own apartment.

"'Why is he yelling at me? Why is he coming at me? Why is the display of my gun not working? He must have a weapon,'" Rogers said Guyger was thinking. "He must want to kill me because I caught him burglarizing my apartment, and he's getting closer.'"

But Hermus said Guyger was preoccupied with text messages from her former lover when she entered the wrong apartment.

Hermus said Jean, 26, didn't stand a chance.

"No opportunity for de-escalation, no opportunity for him to surrender. Bang, bang. Rapid," Hermus told the jury, according to ABC News.

The jury, which is sequestered, will consider whether Guyger is guilty of murder, manslaughter or acquitted of the charges.