DALLAS, Texas (AP) — A white former Dallas police officer who shot her black unarmed neighbour to death after, she said, mistaking his apartment for her own was convicted of murder yesterday in a verdict that prompted tears of relief from his family and chants of “Black lives matter” from a crowd outside the courtroom.

The same jury that found Amber Guyger guilty in the September 2018 death of her upstairs neighbour, Botham Jean, will consider her fate after hearing additional testimony which started yesterday afternoon. She could be sentenced to from five to 99 years in prison under Texas law.

The jury took a matter of hours to convict Guyger, 31, after six days of testimony.

Cheers erupted in the courthouse as the verdict was announced, and someone yelled “Thank you, Jesus!” In the hallway outside the courtroom, a crowd celebrated and chanted “black lives matter”. When the prosecutors walked into the hall, they broke into cheers.

After the verdict was read, Guyger sat alone, weeping, at the defence table.

Jean’s friends and family testified yesterday at the punishment phase of the trial. First on the stand was Allison Jean, who said her son was killed just before he was due to turn 27.

“My life has not been the same. It’s just been like a roller coaster. I can’t sleep, I cannot eat. It’s just been the most terrible time for me,” she said.

Guyger’s defence attorneys can argue that she deserves a light sentence because she acted out of sudden fear and confusion. The judge is expected to provide guidance on sentencing law.

It is unclear how long the punishment phase of the trial will last.

The basic facts of the unusual shooting were not in dispute throughout the trial. After a long shift at work and still in uniform, Guyger walked up to Jean’s apartment — which was on the fourth floor, directly above hers on the third — and found the door unlocked. Thinking the apartment was her own, she drew her service weapon and entered.

Jean, an accountant from the Caribbean island nation of St Lucia, had been eating a bowl of ice cream when Guyger entered his home and shot him.

The shooting drew widespread attention because of the strange circumstances and because it was one in a string of shootings of unarmed black men by white police officers.

“A 26-year-old college-educated black man, certified public accountant, working for one of the big three accounting firms in the world…it shouldn’t take all of that for unarmed black and brown people in America to get justice,” Benjamin Crump, one of the lawyers for Jean’s family, said at a news conference yesterday.

Crump said the verdict honours other people of colour who were killed by police officers who were not convicted of a crime.

The verdict may have defused tensions that began simmering Monday when jurors were told they could consider whether Guyger had a right to use deadly force under a Texas law known as the castle doctrine — even though she wasn’t in her own home.

The law is similar to “stand your ground” measures across the US that state a person has no duty to retreat from an intruder. Prosecutor Jason Fine told jurors that while the law would have empowered Jean to shoot someone barging into his apartment, it doesn’t apply “the other way around”.

Guyger was arrested three days after the killing. She was later fired and charged with murder. Tension has been high during the trial in Dallas, where five police officers were killed in an attack three years ago.

Guyger tearfully apologised for killing Jean and told the jurors she feared for her life upon finding the door to what she thought was her apartment unlocked. Guyger said Jean approached her when she entered the unit with her gun out. Prosecutors suggested he was just rising from a couch toward the back of the room when the officer shot him.

In a frantic 911 call played repeatedly during the trial, Guyger said, “I thought it was my apartment” nearly 20 times. Her lawyers argued that the identical physical appearance of the apartment complex from floor to floor frequently led to tenants going to the wrong apartments.

But prosecutors questioned how Guyger could have missed numerous signs that she was in the wrong place, asked why she didn’t call for backup, and suggested she was distracted by sexually explicit phone messages with her police partner.