The departmental trial for the police officer who shot and killed unarmed Bronx teen Ramarley Graham started Tuesday.

Graham, 18, was killed in February 2012 after he was chased by officer Richard Haste into his Bronx apartment and shot as he tried to flush marijuana down a toilet. Graham’s younger brother and grandmother were in the apartment at the time.

The departmental trial follows a series of grand-jury actions in state court and a federal investigation that concluded without any charges filed. Haste faces possible firing from the NYPD, but any discipline recommendation would ultimately be up to Commissioner James O’Neill to approve.

Just before the shooting, Haste yelled that he thought Graham had a gun. Several officers in Haste’s unit that were present that day testified on Tuesday that they saw a gun in Graham’s waistband.

In her opening statement, Beth Douglas of the Department Advocate’s office didn’t dispute that Haste perceived there was a gun and danger, but rather argued that he should have secured the scene and called for specialized backup, like the Emergency Services Unit.

“There was no emergency circumstances that would [justify] the breaking down of that door,” said Douglas, who is the prosecutor in this case. Protocol, she added, would be to “isolate, contain, and call for backup.”

The high-risk entry requires a “slow and methodical approach,” Douglas said, adding: “The tactical failures of Police Officer Haste rest solely on him.”

Stuart London, Haste’s attorney, opened his argument Tuesday by painting the cop as one of many officers in the drug-enforcement unit he was part of at the time, re-directing responsibility to others. London also emphasized on what he depicted as the limited specialized training Haste and others received on the SNEU team, or Street Narcotics Enforcement Unit.

“It’s a dangerous, dangerous job,” said London, who added that Haste’s “state of mind was shaped” by the fact that he thought Graham had a gun. He said it was Haste’s partner who made the decision to forcibly open the door to Graham’s second-floor apartment.

“Police work should not be subject to second guessing. The officers have to make a split-second decision on what to do,” London added. “He thought he was about to be shot.”

Constance Malcolm, Graham’s mother, said she’s been “fighting for justice” for nearly five years.

“I’m not going to stop fighting. My purpose is to make sure Richard Haste is fired,” she said, standing outside police headquarters. “This man murdered my son for no reason. He broke into my home... I’m not asking for anything special, I’m just asking for justice.”