A San Mateo County sheriff’s deputy shot and killed an 18-year-old woman after family members called 911 seeking an ambulance.

The family members sought medical help because the dead woman, Yanira Serrano-Garcia, had psychological disabilities and had been refusing to take her medication, according to local television station KRON.

The incident occurred on Tuesday around 9:30 p.m. near the coastal Bay Area city of Half Moon Bay.

Serrano-Garcia was in the street outside the Moonridge housing complex when armed deputies — not paramedics — arrived. Witnesses and the deputies say she had a knife.

“Shortly thereafter there was a confrontation where the deputy was in fear for his life and as a result he fired his weapon,” sheriff’s department spokeswoman Rebecca Rosenblatt told KRON.

A 12-year-old witness to the deadly confrontation, Dyanna Ruiz, explained that she was on her way to visit a friend when she saw Serrano-Garcia holding what looked like a knife and chasing an unidentified deputy.

“I saw the girl running at him with something in her hands,” Ruiz said, according to SFGate.com. “I didn’t know what was happening. I was really scared about what to do.”

The deputy shouted at Serrano-Garcia to stop. She didn’t. It was perhaps out of concern for the 12-year-old girl’s life that the deputy then shot and killed Serrano-Garcia.

Serrano-Garcia’s grieving relatives say they don’t understand why it was necessary for the unnamed cop to use deadly force.

“She has special needs and we just want answers,” Tiny Serrano, Serrano-Garcia’s brother said. “Who are we supposed to call now when we need help when who is supposed to help us is killing our kids?”

Another relative, Lorenzo Serrano, said the family is demanding answers.

“We want to know what happened,” he told SFGate. “Why didn’t (the deputy) use a Taser? I don’t know. There’s many questions in the air. We just want to know what really happened and we want justice, basically.”

The sheriff’s department has placed the unnamed deputy on paid leave. Sheriff’s detectives and the local district attorney are investigating the incident.

Follow Eric on Twitter and on Facebook, and send story tips to erico@dailycaller.com.