HALF MOON BAY -- More than two weeks after Yanira Serrano-Garcia was fatally shot by a deputy on the street where she lived, her 29-year-old brother can't stop thinking about what could have gone differently.

A slight shift in judgment, and maybe the trigger wouldn't have been pulled. A split-second difference, and maybe his only sibling would still be alive.

"My brain can't comprehend what happened to my little sister. How can a call for help turn into something so tragic?" said Tony Serrano-Garcia. "It's a nightmare I don't wish for anyone to experience. Seeing someone you love so much just ... gone in 20 seconds. It's just too much."

Yanira Serrano-Garcia, 18, who had a history of mental illness, was shot and killed about 9:23 p.m. by sheriff's deputy Menh Trieu, a nine-year veteran of the San Mateo County Sheriff's Office.

Sheriff's spokeswoman Rebecca Rosenblatt said Trieu has said he fired because he feared for his life, and the office is standing by him.

While the San Mateo County District Attorney's Office continues to investigate the shooting as it does with all officer-involved shootings, the family and its attorneys are calling for a criminal investigation into Trieu's conduct, followed by a Town Hall hearing to present the DA's findings to the community. They are also prepared to file a lawsuit.

"Officers are required to take a look at the whole circumstance when they arrive on scene," attorney Jonathan Melrod said. "Was he really afraid of a slight, 18-year-old girl who was not even able to run? Or did he pull the trigger without adequate attempts to deescalate the situation?"

Family members say Serrano-Garcia was obese and walked with a limp due to a birth defect.

Rosenblatt said there is a "lot of conjecture flying around" about the shooting. The sheriff's office is awaiting the district attorney's report, which she said should put to rest the speculation.

A legitimate threat?

Trieu responded alone June 3 to the Moonridge Housing Complex off Miramontes Point Road and found Serrano-Garcia in the road with a kitchen knife, sheriff's spokeswoman Rebecca Rosenblatt said. A 12-year-old witness told reporters that Trieu yelled at the teen when he saw her coming toward him and opened fire when she kept running, an account District Attorney Steve Wagstaffe said she also gave investigators.

The teen, who had just been admitted to a continuation high school and was set to participate in a job placement program, was pronounced dead at the scene.

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.

Of the six fatal officer-involved shootings since 2010, four involved patients who were mentally ill, authorities said.

While Chang was killed the first time his parents resorted to calling police, Tony Serrano-Garcia said the family had called police about five times before to help de-escalate Yanira's mental health crises. Each time authorities sent an ambulance and medical crews to either talk her down or take her to the hospital.

But that wasn't the case on the evening of June 3.

Closest responder

Tony Serrano-Garcia said his sister was about to start a new medication and that he trusted mental health experts to supervise his sister during the initial onset. His sister often abandoned her medication due to unpleasant side effects like weight gain and a loss of interest in things she once loved and stressed to her brother that she "just wanted to be normal again," playing soccer and spending time with her friends.

He and his mother decided to call an ambulance June 3 when she once again refused to take her medication and was acting increasingly agitated. But this time a sheriff's deputy showed up from the substation just yards from the Serrano-Garcia home.

While Wagstaffe did not have an immediate conclusion as to why the response was so different from before, he said Trieu was likely the closest responder to the scene.

Wagstaffe said he hoped to complete his investigation within eight weeks of the shooting and that toxicology reports and forensic analysis of both the knife and the firearm are still pending.

As for analysis of training and tactics, Wagstaffe said policies and protocols regarding the treatment of mentally ill subjects are reviewed and re-examined every time a loss like Yanira's rocks the community.

Trieu had not undergone CIT training, a 40-hour state-certified curriculum focusing on mental illnesses, psychiatric drugs, suicidal behavior and a host of other issues.

"There is no process that exists that can't be looked at whenever there's a tragedy," said Wagstaffe, who himself teaches at the county's CIT academies. "San Mateo County has been a leader in its concern about this topic, and we will always take a look to see if there's anything we can do better.

"In the end, the public is going to have full confidence in the fairness of the process. I am confident of that."

Meanwhile, community members on June 17 called on the City Council to establish June 3 as Yanira Serrano-Garcia Memorial Day.

"Whatever the outcome is, it's not going to bring my sister back," Tony Serrano-Garcia said. "It's not going to fill that hole in my heart. It's not going to take away the feeling of emptiness for my family. But we want justice ... and we want this to never, ever happen again."

Contact Erin Ivie at eivie@bayareanewsgroup.com. Follow her at Twitter.com/erin_ivie.