​​A Brooklyn grand jury began hearing evidence Wednesday against an NYPD officer who accidentally fired his gun and killed an innocent man in the darkened stairwell of an East New York housing project last year, sources told The Post.

“They finally got their full complement of grand jurors [Tuesday] and they started [Wednesday],” a court source said.

The source noted that picking a panel to weigh charges against Police Officer Peter ­Liang in the fatal shooting last November of Akai Gurley, 28, was difficult because the jurors were told they would be hearing evidence for two to three months.

A police source said cops who responded to the scene have been called to Brooklyn Supreme Court to testify about the shooting and that Liang’s partner is expected to testify this week.

District Attorney Ken Thompson said in December that he would convene a grand jury to hear evidence against Liang.

Liang defense attorney Stephen Worth and a DA spokeswoman both declined to comment.

Police Commissioner Bill Bratton called the unarmed Gurley a “total innocent.” Liang, 27, has said he fired his gun accidentally.