HALF MOON BAY — An 18-year-old woman was fatally shot by a sheriff’s deputy Tuesday night after she lunged at him with a knife as he encountered her in the street, authorities said.

The woman, identified as Yanira Serrano-Garcia, of Half Moon Bay, was shot and killed by sheriff’s deputy Menh Trieu, with whom she got “in a confrontation” about 9:23 p.m., San Mateo County Sheriff’s spokeswoman Rebecca Rosenblatt said.

Two deputies responded to the Moonridge Housing Complex at Miramontes Point Road, east of Highway 1, where the woman lived, Rosenblatt said. The family told dispatchers the woman was mentally ill and was located down the block with a knife, and that she refused to put the weapon down when asked.

While details of what happened between deputies’ arrival and the shooting are still emerging, Rosenblatt said the deputies found the woman standing in the street and that she came at them with a knife about 20 seconds later. The threat prompted Trieu to fire at least one shot at the woman, she said The other deputy did not shoot.

Emergency crews responded and pronounced the woman dead at the scene. While Rosenblatt could not confirm where on her body the woman was shot, she noted Serrano-Garcia was not shot in the back as some news agencies reported.

Trieu, a nine-year veteran of the San Mateo County Sheriff’s office, was sequestered and taken into interviews with investigators, Rosenblatt said. He has been placed on paid administrative leave, per department protocol.

Groups of emotional family members came and went from the family home Wednesday morning, with audible cries that could heard from the sidewalk of the housing complex. Family members said the woman had gone off required medication and was acting erratically in the hours leading up to the shooting.

“I don’t know why they couldn’t have done better things instead of getting a bullet through her and trying to shoot her,” said Saul Miramontes, Serrano-Garcia’s cousin. “She was kind of sick — you know, at least they could have Tasered her or at least tackled her.”

The incident marked the second officer-involved shooting in San Mateo County this year, making 2014 the first year since 2006 with more than one officer-involved shooting. The last officer-involved shooting occurred March 18 in Pacifica, when 34-year-old Errol Chang, who was also mentally ill and armed with a knife, was shot and killed by a Daly City SWAT officer.

District Attorney Steve Wagstaffe, who investigates all officer-involved shootings in the county, released a statement Friday that the shooting of Chang was “justified” and he would not be pressing charges. He called the officers’ response to the incident “exemplary,” and said the community had been “well served by their actions.”

The joint investigation into Serrano-Garcia’s death is ongoing between Wagstaffe’s office and the San Mateo County Sheriff’s office. An autopsy is pending at the San Mateo County coroner’s office.

Follow Erin Ivie at eivie@bayareanewsgroup.com.