DALLAS >> A white Dallas police officer went on trial Monday in the shooting death of a black neighbor as attorneys sparred over whether the officer was distracted by a phone call when she mistook the neighbor’s apartment for her own and the victim for an intruder.

Prosecutors contend Amber Guyger, 31, was distracted by the conversation with a colleague with whom she had a sexual relationship. Guyger’s attorneys argued that she fired in self-defense based on the mistaken belief that she was in her home and that Botham Jean was a burglar.

Jean, a 26-year-old accountant from the Caribbean nation of St. Lucia, “was doing no harm to anyone, which was his way,” Dallas County Assistant District Attorney Jason Hermus said in an opening statement.

Jean was in his living room eating a bowl of vanilla ice cream on Sept. 6, 2018, when Guyger entered the apartment, which was one floor directly above her apartment, Hermus said.

Hermus said Guyger had worked overtime that day, mostly involving office work that was not strenuous. He said jurors will see sexually explicit messages that Guyger exchanged that evening with a co-worker that discussed meeting up after her shift ended. He said some messages had been deleted from Guyger’s phone after the shooting.

During pretrial proceedings, prosecutors and defense attorneys clashed over whether those messages should be entered into evidence.

In his opening statement, defense attorney Robert Rogers rejected the prosecution argument that there were unique signs that would have signaled to Guyger that she was on the wrong floor. In fact, he said, the identical look of the apartment complex from floor to floor often led to confusion among tenants, with dozens regularly parking on the wrong floor or attempting to enter the wrong apartment.

Rogers said the floors of the parking garage were not clearly marked so it was understandable when Guyger, tired from a long shift, pushed open a door and believed an intruder was inside.

Guyger “was on autopilot,” he said of her entrance to Jean’s apartment. “She had tunnel vision.”

Rogers also dismissed as “preposterous” the relevance to Jean’s death of Guyger’s sexual relationship with her partner.

Martin Rivera, Guyger’s colleague at the time and her former lover, acknowledged having a 16-minute telephone conversation with Guyger as she headed home from work the night of the shooting. He said the two exchanged sexually explicit messages and images earlier that day but denied making plans to rendezvous with Guyger later that night, as prosecutors suggested.

When prosecutors asked River what the conversation was about, he said he believed it was mostly about police work but his memory of the call was hazy.

After the shooting, prosecutors said, Guyger deleted the logs of her text exchanges with Rivera from her cellphone. Rivera acknowledged doing the same thing.

Earlier in the day, Allisa Findley of New York, Jean’s older sister, testified that she was the first in the family to be informed of his death via a phone call from a hospital social worker.

“I just immediately became cold,” Findley said. “It just didn’t make sense.”

Findley said she broke the tragic news to their mother and that she still sometimes calls her brother’s phone hoping that he will answer.

“I haven’t accepted it yet,” she said.

The case is being heard by a jury that appeared to have a majority of women and people of color.

The shooting attracted intense national scrutiny for the strange circumstances and because it was one in a chain of shootings of unarmed black men by white police officers.

The trial’s outcome may hang on whether the jury believes that Guyger’s mistake was reasonable, according to legal experts.

Guyger was off duty but still in uniform when she shot Jean. She told investigators that after a 15-hour shift she parked on the fourth floor of the complex’s garage — rather than the third floor, where she lived — and found the apartment’s door ajar.

Three days later, Guyger was arrested for manslaughter. She was subsequently fired from the Dallas Police Department and charged by a grand jury .

The jury will have to decide whether Guyger committed murder, a lesser offense such as manslaughter or criminally negligent homicide, or no crime at all.