The former Dallas police officer who fatally shot an unarmed black man in his apartment was distracted by a phone call with a lover, a prosecutor said Monday.

Attorneys for Amber Guyger, who is white, said she tried to defend herself after mistaking the apartment for her own and thinking Botham Jean was a burglar.

The accountant from St. Lucia never posed a threat to Guyger, said Dallas County Assistant District Attorney Jason Hermus, and he was eating a bowl of ice cream in his living room when she entered on Sept. 6, 2018.

Hemus told jurors Guyger missed signs she was on the wrong floor in the apartment complex, suggesting she was upset after a phone call she had minutes before the shooting. Guyger had spoken with a co-worker whom she had a sexual relationship with, Hemus said.

Defense attorney Robert Rogers dismissed the relationship as irrelevant to the murder trial. Dozens of other tenants in the building, he added, regularly parked on the wrong floor and tried to enter the wrong apartment. Different floors looked identical, he said.

Tired from a 15-hour shift, Rogers said it was understandable how Guyger didn't notice she was on the wrong floor and believed her neighbor was an intruder.

Botham Jean's death prompts the question:When will it be safe to exist while black in US?

According to a 911 call first obtained by Dallas TV station WFAA, Guyger apologized to Jean, 26, while reporting the shooting. She can be heard saying “I’m gonna lose my job” and “I am going to need a supervisor" in the six-minute recording.

The Dallas Police Department fired Guyger, now 31, in September. Authorities initially arrested her for manslaughter, but a grand jury indicted her in November 2018 on a murder charge.

The jury in the murder trial that began Monday will decide whether Guyger committed murder, a lesser offense such as manslaughter or criminally negligent homicide, or no crime at all.

Contributing: The Associated Press