A Purcellville police officer who fatally shot an apparently suicidal teenager acted with justifiable force, Loudoun County Commonwealth’s Attorney James E. Plowman said Thursday.

The officer, identified by authorities as T. Hood, responded on May 24 to a report of a teen armed with a knife and threatening to kill himself. Just seconds after Hood encountered Christian Alberto Sierra, the 17-year-old advanced on the officer, ignoring commands to stop and drop his knife, Plowman said.

“The officer was justified in his use of lethal force,” Plowman said at a news conference in Leesburg. “He was faced with an immediate and real threat.”

On Thursday, authorities also released a report summarizing the findings of a Virginia State Police investigation of the fatal shooting, the first in the history of the 15-member Purcellville police department.

Sierra’s death stunned Purcellville, a small western Loudoun community, and some residents questioned the officer’s use of lethal force — a topic that has received increased national scrutiny after the high-profile police shooting in August of an unarmed African American teen, Michael Brown, in Ferguson, Mo.

Wayne Cohen, an attorney for Sierra’s family, said the family was “disappointed by the findings” and would pursue other avenues for justice.

“They hope to have an independent investigation conducted where the investigators are not affiliated with the Commonwealth of Virginia,” Cohen said. “The reality is that Christian had a knife, a police officer shows up with a gun, and a teenager’s life was lost. Those facts deserve independent, detailed investigation to try to figure out why the situation was not de-escalated before shots were fired.”

Sierra, a junior at Loudoun Valley High School, was socializing with several friends at a home on Frazer Drive in Purcellville shortly before he was killed, according to authorities. While watching a movie, he appeared to grow depressed, telling his friends that he believed in reincarnation and wanted to kill himself, the report says.

The teen then retreated to a bathroom, where he began cutting and stabbing his neck with a seven-inch knife, according to the report. One of Sierra’s friends tried to take the knife from him, called 911 and pursued him into the yard outside the house, where the struggle over the knife continued, the report says.

Three officers responded to the 911 call, including Hood, who encountered Sierra and the friend on Frazer Drive and saw that both were wounded and bloody, Plowman said at the news conference.

“Within seconds . . . [Sierra] breaks free from the struggle with his friend and immediately goes at the officer,” Plowman said.

After Hood ordered Sierra to stop, the teen continued to advance, and Hood fired a shot that struck the teen in the chest, according to the report. When Sierra continued to approach the officer, Hood fired three more times, wounding him again in the chest, the report says.

Hood had retreated about 10 to 15 feet from Sierra before firing, Plowman said.

Sierra was pronounced dead by emergency medical personnel, the report says. Hood, a Marine Corps veteran, had been with the Purcellville police for 16 months at the time of the shooting. The officer was placed on routine administrative leave pending the outcome of the investigation.

Purcellville Police Chief Darryl Smith said Thursday that Hood would return to duty within a couple of weeks.

The 16-page report also contains a history of Sierra’s previous encounters with Purcellville police. Between March 2009 and March 2014, officers went to the family home more than a dozen times in response to range of complaints, including that Sierra was behaving in a “difficult” or violent way toward his parents, was refusing to take prescribed medication or was refusing to go to school, the report says.

Plowman said Hood had encountered Sierra once before, in June 2013, when Hood went to the home after Sierra’s father contacted police to express concern for the safety of the teen’s mother. Sierra had pushed his mother and taken her cellphone, the report says, but his parents decided to take no further action against him.

Plowman said he did not know whether Hood recognized Sierra during the May 24 incident. Plowman also said crisis-intervention training, which teaches law enforcement officers techniques for handling people with mental health problems, might not have been of help in a situation involving an “immediate threat of violence.”

Plowman said his office met with Sierra’s family before the report was released. “I’m not sure words can describe someone’s reaction to the death of their child,” he said.