DALLAS (Reuters) - A Dallas police officer missed clues, including the smell of marijuana, when she entered an apartment she believed was her own and shot dead a man eating a bowl of ice cream, a prosecutor said Monday at the start of the former officer’s murder trial.

Amber Guyger, who is white, has told investigators she mistook 26-year-old Botham Jean, a black man, for a burglar after she mistakenly entered his central Dallas apartment one floor above her own.

A defense attorney said in his opening statement that the officer was so exhausted she was “on autopilot” and that dozens of other people who live in the same apartment complex reported they had confused floors because all of them are virtually identical.

Assistant District Attorney Jason Hermus told the jury of four men and 12 women that Guyger had a 16-minute phone conversation with her former partner, with whom she had a romantic relationship, on the way home from work that night after a 13-1/2 hour shift.

“Whatever is on her mind after that conversation has consumed her attention entirely,” Hermus said in his opening statement.

Defense attorney Robert Rogers told jurors the shooting was a tragic mistake. He also said, without indicating where he obtained the data, that 93 tenants in the same complex said they had at least once mistakenly pulled onto the wrong floor in the parking garage.

“Was it evil of her not to count the floors?” Rogers said about Guyger parking on the wrong level in the garage.

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He later added: “She knew she had made a terrible mistake, but it was not out of evil.”

The shooting, one of a series of high-profile killings of unarmed black men and teens by white U.S. police, sparked street protests, particularly because prosecutors initially moved to charge Guyger, 31, with manslaughter, a charge for killing without malice that carries a lesser sentence than murder.

In contrast to cases like the killings of Michael Brown in Missouri and Philando Castile in Minnesota, Guyger shot Jean, a PwC accountant who was a native of the Caribbean island nation of Saint Lucia, while she was off duty, rather than while responding to a reported crime.

Hermus told the jury that when Guyger got to her apartment complex, she parked on the fourth floor instead of the third, where she had lived for two months.

When she arrived at what she thought was her unit, she failed to notice the bright red semi-circle welcome mat in front of Jean’s apartment, he said.

Jean’s apartment was also unlocked, messy and smelled of marijuana, three more signs that should have tipped Guyger off that it was not her apartment, Hermus said.

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Despite the clues, she still burst through the door and opened fire, striking Jean once in the chest as he watched television and ate a bowl of vanilla ice cream.

“He was in the sanctuary of his home doing no harm to anyone,” Hermus said. “There he lie on his back in his home bleeding to death alone with his killer.”

Guyger then called 911.

“‘I shot a guy thinking it was my apartment,’” Hermus said, recounting what Guyger told the dispatcher, noting that she never said that Jean posed a threat to her.

Before opening statements began, state District Judge Tammy Kemp sequestered the jurors, shielding them from possible outside influence and local news coverage of the case.

Kemp also denied a motion to exclude evidence from Guyger’s texts and phone calls and a motion for a mistrial from Guyger’s attorneys.

The district attorney's office re-examined the case after the public protests, and a grand jury in late November indicted Guyger on murder charges, with the maximum punishment of life in prison. reut.rs/2mxclAB

After the incident, she was initially placed on administrative leave but was fired days later.