Amber Guyger fatally shot Botham Jean in his apartment in Dallas on 6 September a year ago. Guyger said that going through the doorway with her pistol drawn, "was the only option that went through my head".
Guyger, who is White, was sacked from the Dallas Police Department after being indicted for fatally shooting Jean, 26, on September 6, 2018, a Black man and accountant who hails from the Caribbean island of St. Lucia.
A Texas jury was set to continue deliberations on Tuesday in the trial of a white former police officer accused of murdering a black neighbour in his own apartment.
A sob, which sounded like it came from Guyger's bench, cut the judge off and Kemp paused to address the courtroom. She now faces life in prison, although appeals are expected.
The verdict comes after a tense seven-day trial in which Guyger admitted: "I shot an innocent man".
"I ask God for forgiveness, and I hate myself every single day", Guyger said before the jury last week, her voice shaking.
The basic facts of the unusual shooting were not in dispute throughout the trial. When she arrived, Guyger got out of her vehicle, then entered Jean's apartment, firing at the man soon after and hitting him in the chest. Her lawyers said she was in uniform and had just finished a 13-hour workday when she opened Jean's door.
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She was initially arrested several days after the killing and charged with manslaughter before being released from jail that same day.
Guyger, who admitted under cross-examination that she meant to kill Jean when she shot him, was facing up to 90 years in prison.
Assistant District Attorney Jason Hermus said: "Killing this man was unnecessary and it was unreasonable".
Guyger then called 911 and told the operator over and over: "I thought it was my apartment". (His door had not been secured.) Guyger said she believed that Jean was an intruder in her apartment, and she was in fear for her life.
But Guyger, a white woman who had just finished a long shift at work, was on the wrong floor of the building. Judge Tammy Kemp reignited the debate on Monday after deciding the jury could consider the state's "Castle Doctrine", a legal exception that says a person in their home is not required to retreat before using deadly force on an intruder.
"We're still dealing in America with black people being killed in some of the most arbitrary ways, driving while black, walking while black, and now we have to add living while black", civil rights attorney Benjamin Crump, one of the Jean family attorneys, said at the time.
Neither prosecutors nor the defence focused on race during the trial. His murder drew widespread attention because of the unusual circumstances and because it was one in a string of shootings of unarmed black men by white police officers. But unlike other recent high-profile killings, such as those of Michael Brown in Missouri and Philando Castile in Minnesota, Guyger was not on duty or responding to a reported crime when she pulled the trigger. Guyger lived one floor directly below Jean.