On Monday, the trial of Officer Liang began before Justice Danny K. Chun, and Ms. Lopez was called as the second witness. Officer Liang, 28, is charged with manslaughter in the fatal shooting of Mr. Gurley, 28, who was walking unarmed with Ms. Butler in the stairwell of the housing project in Brooklyn.

“Akai Gurley is dead today because he crossed paths with Peter Liang, who is sitting here,” Marc J. Fliedner, a prosecutor who is the chief of the civil rights bureau at the Brooklyn district attorney’s office, said in his opening statement, which at one point found him on his knees, demonstrating how Ms. Butler had bent over Mr. Gurley’s body.

Both sides agree that while Officer Liang was on patrol on Nov. 20, 2014, he fired his gun down an unlit stairwell, and the bullet killed Mr. Gurley.

The prosecution argues that Officer Liang was reckless and violated his training by having his gun out, pulling the trigger with no reason, and neglecting to help Mr. Gurley once he realized the man had been shot. The officer was “worried only about himself,” Mr. Fliedner said, adding, “Instead of calling for help he just stood there, whined and moaned about how he would get fired.”