A 9-year-old boy calmly told of hearing screaming inside his mother’s South Side apartment on the night of July 7, 2011.

“What did you hear?" asked prosecutor Marilisa Janssen.

“Screaming,” the child said.

“Who did the screaming?” asked Janssen.

“My mom,” the child replied.

His mother’s boyfriend, Gary Cooper, 25, was stabbed to death by the child’s father, Richard Mendez. He pleaded guilty to the murder last February and is serving a 45-year prison term.

Friday’s testimony was part of the trial of Mendez’s brother, Jacob Gonzalez.

Gonzalez is charged under the law of parties. Though he did not kill Cooper, prosecutors say he aided and encouraged the slaying.

Gonzalez and Mendez broke into the apartment of Ashley Mendez, Mendez’s estranged wife, and attacked Cooper, who she testified earlier was her boyfriend.

The child said that after hearing the screams, he opened the door to the bedroom where he and his two brothers were sleeping.

“Did you see anybody fighting with their hands?” asked Janssen.

“No,” the child said.

When asked whether he saw anyone with an object in their hand, he said his father was holding something but he did not know what he was holding.

The child said he retreated to the bedroom and when he came out a little later, he saw blood on the floor.

Cooper, who was stabbed in the chest, staggered from the apartment and collapsed and died in the parking lot.

Testimony will continue on Monday in 144th District Court. Senior Judge Pat Priest is presiding over the trial.

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

For a list of recent stories Paul Venema has done, click here.