(CNN) A second suspect has been arrested in the killing of Joshua Brown, a key witness in the murder trial of a former Dallas police officer convicted of shooting their neighbor in his own apartment.

Brown testified against former officer Amber Guyger who fatally shot their neighbor, Botham Jean. Guyger said she killed the 26-year-old accountant after she mistakenly walked into his apartment, one floor above her unit, and mistook him for a burglar.

Ten days after testifying for the prosecution in the high-profile trial, Brown was shot dead Friday during a drug deal gone wrong, Assistant Chief Avery Moore said.

He was killed after an argument with one of three Louisiana men who had met him in Dallas for a drug purchase, Moore told reporters.

Police identified the men as Thaddeous Green, Jacquerious Mitchell and Michael Diaz Mitchell.

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