Amber Guyger had been a police officer for four years and was fired following the shooting

A former Texas policewoman went on trial Monday for shooting dead a neighbor in his home last year in what her lawyer claimed was a "tragic mistake."

The case has inflamed racial tensions since former Dallas, Texas police officer Amber Guyger, 31, who is facing murder charges, is white, while the victim, Botham Jean, was black.

Guyger claimed after the September 6, 2018 shooting that she mistakenly believed she had returned to her own apartment and that the 26-year-old Jean was an intruder.

In fact, Guyger had entered Jean's unlocked apartment located in the same building but one floor above her own home.

Guyger, who was off-duty at the time after working a nearly 14-hour shift, lived in apartment 1378 on the third floor of the building, while Jean lived in apartment 1478 directly above her on the fourth floor.

"She knows she's made a tragic mistake, but it's not out of evil," Guyger's lawyer, Robert Rogers, told the court, describing it as "human error" and an act of "self-defense."

Guyger, who had been a police officer for four years and was fired following the shooting, "thought firmly and reasonably that she was in her apartment," Rogers said.

Prosecutor Jason Hermus countered that Jean, a native of the Caribbean island of St. Lucia who worked for an accounting firm, "paid the ultimate price" for her errors.

"She walks past 16 different apartments and fails to register the number four on any one of them," Hermus said.

Guyger was initially charged with manslaughter, but a grand jury indicted her on the more serious charge of murder following public protests in Dallas.

Guyger could face life in prison if convicted.