The fatal shooting of a teenager by New Bedford police officers was deemed justified by ­investigators, Bristol District Attorney C. Samuel Sutter ­announced Tuesday.

Officers shot Malcolm ­Gracia, 15, after he stabbed ­Detective Tyson Barnes several times during what police ­described as a routine inter­action between Gracia, his friend, and officers in the Police Department’s gang unit in the Temple Landing housing development.

In a summary of the investigation, conducted by State ­Police and Sutter’s office, investigators determined that Gracia had suffered from a mental illness, was suicidal, told friends he made “a pact with the devil,” and blamed police for his ­father’s death. The report also concluded that Gracia, after stabbing Barnes, would not submit to police.

“It is our conclusion, taking into consideration all of the ­aspects of this incident and mindful of how rapidly events proceeded after the stabbing of Detective Barnes, that this shooting, although tragic, was justified under the circumstances,” the report concluded.

The New Bedford chapter of the NAACP had sought an independent investigation into the shooting in June, citing mistrust between police and the public. Some witnesses said at the time that Gracia was shot as he ran from officers, causing the NAACP to question whether deadly force was necessary. The New Bedford NAACP chapter did not return a call Tuesday.

According to the investigator’s final report, four officers were watching a live video of a Temple Landing basketball court on May 17 and left the Rockdale Avenue police headquarters after seeing two unknown people exchange what “appeared to be a gang handshake” with others. Two detectives approached Gracia and another boy, 16, but the two boys got tense, picked up their pace, and said nothing, the ­report said. They also ignored the officers’ request they take their hands out of their pockets.

After a chase, Barnes caught up to Gracia and grabbed him by the shoulders. That’s when, according to the report, Gracia used a hunting knife with a gutting hook to stab him in the chest, abdomen, and arm, accord­ing to the report.

Barnes fell, and Gracia, still wielding the knife, ran toward another detective. Gracia ­ignored commands and a Taser missed him, the report said.

The detectives fired three shots at Gracia, who fell. The detectives ordered him to stay down, but, still holding the knife, he began to get up. Officers then fired three more times, the report said.

The report included an ­account by an eyewitness who spoke of hearing detectives tell Gracia to stay down.

Gracia’s autopsy revealed that he had been shot four times: twice in the lower back, once in the upper back, and in his head. The examination found marijuana in his system.

The investigative report also recounts events leading up to May 17, offered as a glimpse ­into Gracia’s state of mind. Four days earlier Gracia had posted a photo of himself holding the knife on his Facebook page with the caption “ready to kill.”

A friend told investigators that Gracia blamed police for his father’s death and “that’s why he hated them.” Gracia’s ­father had been fatally shot by Dartmouth police in 2009. Two years later, Gracia had threatened to kill a police officer by shooting him three times, the report says.

Gracia’s companion on the night of the his death also told police that Gracia told him he had “sold his soul to the devil.” Investigators also learned that Gracia had received psychiatric treatment and was prescribed medication that he had not been taking.

“In hindsight, it would have been easy to predict that any ­individual with a mental illness, unmedicated, possessing obsessively conspiratorial thoughts, and a deep animosity toward the police, who has stated he is ‘ready to kill’ and is armed with a knife presented an imminent danger to a police officer,” the report read.