Officials say Mhai Scott acted oddly while handing out pizza samples at this Costco in Sterling. Loudoun deputies responded and fatally shot her after she approached them with a knife, police said. (Dayna Smith/The Washington Post)

A Loudoun County sheriff’s deputy who shot and killed a woman holding a knife at a Costco in Sterling acted in justifiable self-defense, Loudoun County Commonwealth’s Attorney James E. Plowman announced Wednesday.

Plowman offered new details for the first time of a dramatic showdown in May between the woman and sheriff’s deputies, who he said were forced to make a split-second decision in a busy store with shoppers nearby. The woman, Mhai Scott, 38, charged at deputies while holding a knife and scissors, and they tried to stun her with a Taser weapon before one resorted to deadly force, he said.

“The deputy’s actions constituted a justifiable homicide . . . undertaken in self-defense and the defense of others,” Plowman said.

The announcement at a news conference in Leesburg followed a months-long investigation into the shooting, which had prompted an outcry from Scott’s relatives — and from members of the Filipino community, who argued that the Filipino woman had been the victim of unnecessary force.

But after reviewing voluminous evidence, including numerous interviews, police reports, surveillance video footage, photographs and extensive background and medical records, Plowman said he found that the deputy’s actions were appropriate. He noted that the use of fatal force must be considered from the perspective of what seemed reasonable to the officer on the scene — “not Monday-morning quarterbacking” with the benefit of hindsight.

The shooting took place about 3 p.m. May 29 after sheriff’s deputies responded to a report of a disorderly person at the Sterling store. When the deputies arrived, they found Scott, a food server at Costco, behaving erratically and wielding a 13-inch knife and a pair of scissors, authorities said.

Authorities initially said that two deputies were directly involved in the encounter, but the investigation summary report named five deputies who confronted Scott as she stood near the center of a store aisle. Three deputies approached Scott from one side of the aisle and two from the other, the report said.

Sheriff’s office officials said that Scott did not respond to commands to drop her weapons and that the deputies were unable to subdue her with the Taser weapon. Plowman said Wednesday that it remained unclear whether the stun gun failed because of mechanical problems or human error; he said the equipment would be analyzed.

According to the report, Scott showed no reaction to the Taser discharge but instead turned toward the three deputies at one end of the aisle and charged at them with the knife and scissors raised. Then, one of the three deputies — identified as K. Foster in the report — fired five shots, striking Scott four times and killing her. Another deputy was struck by a bullet in the leg, possibly the result of a ricochet, Plowman said.

Foster, who has been on administrative leave since the shooting, will “probably be returning to duty very soon,” Loudoun Sheriff Mike Chapman said Wednesday.

Plowman noted that Scott had a history of psychological issues and had been confronted by law enforcement before. He cited a February 2012 incident in Ohio when Scott’s then-husband contacted police because he feared for the safety of the couple’s two daughters, ages 10 and 7 at the time. When police arrived at the couple’s home, Scott was “yelling incoherent statements about Satan,” and the children were hiding in an upstairs closet, according to the investigation report.

Scott sought psychiatric care in July 2012 after moving to Northern Virginia, according to the report.

Plowman said Wednesday that authorities had met earlier in the day with members of Scott’s family.

“It’s never an easy conversation, and it’s a tragic situation. . . . We sat with them; we answered all of their questions,” Plowman said. “They seemed to understand the situation.”

Scott’s ex-husband has declined to comment on the shooting or the subsequent investigation. Scott’s uncle, who lives in the Washington area, could not be reached for comment.

The Philippine Embassy, Filipino American organizations and members of Scott’s family in the Philippines were among those questioning whether the shooting represented a disproportionate and unnecessary use of force.

Scott’s mother and sister, who live in the Philippines, appeared in an interview aired on a television network in Manila soon after the killing. Speaking in Tagalog, their native language, they both wept and said they were distraught and angry over Scott’s death.

After the announcement Wednesday, Philippine Ambassador Jose L. Cuisia Jr. said in a statement that embassy officials were reviewing the “comprehensive and technical” report.

Ed Navarra, chairman of the National Federation of Filipino American Associations, issued a statement Wednesday in which he expressed disappointment with the outcome of the investigation.

“While we understand that police officers need to use force to protect themselves, we continue to believe that when guns were fired at Mhai Scott, it was an unreasonable and excessive use of force which showed a reckless disregard for the life of an innocent woman,” the statement said. “Mhai’s wrongful death cries out for justice and the Filipino American community will pursue every means to ensure that justice is done.”

Stefanie Dazio contributed to this report.