Though he did not kill the boyfriend of his brother’s estranged wife, Jacob Gonzalez, 24, is on trial charged with murder.

Ashley Mendez, 24, the wife of Gonzalez’s brother, testified Wednesday that Gonzalez and her estranged husband, Richard Mendez, broke into her South Side apartment on the night of July 7, 2011.

She said the pair attacked her boyfriend, Gary Cooper.

Cooper, 25, was stabbed to death.

“Who stabbed Gary?” Prosecutor Marilisa Janssen asked.

“Richard,” Mendez replied.

Mendez pleaded guilty to Cooper’s death last year and is serving a 45-year sentence.

Gonzalez is being tried under what is called the Law of Parties. Though he did not kill Cooper, prosecutors said he “aided and encouraged” the murder and, under the law, can be tried for murder.

If he is found guilty, Gonzalez faces a maximum sentence of life in prison.

Testimony will continue on Thursday before Senior Judge Pat Priest in 144th District Court.

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