Yao’s parents have been dealing with their son’s mental illness for several years, which “undoubtedly” played a part in the tragic incident, according to Carney. He said he will be assembling a team of doctors who will examine Yao’s condition, and will allow him to present evidence of his mental illness.

Victim stabbed 20 times

On Saturday morning, Yao allegedly approached Deane Stryker from the back while she was reading at the Winchester Public Library and, unprovoked, began stabbing her with a 10-inch hunting knife. Stryker was able to get up and make it to the front lobby, while Yao ran after her, continuing to inflict wounds. The autopsy conducted by the Office of the Chief Medical Examiner, determined the cause of Stryker’s death to be “sharp force trauma with 20 knife wounds, according to the statement presented by Assistant District Attorney Kate Cimini.

During the attack Lester Taber, a 77-year-old man who had been sitting near Stryker, yelled at the defendant to stop, according to Cimini’s statement. In reaction, Yao inflicted a cut on Taber’s upper arm with a knife. Several individuals managed to separate Yao from Stryker and cornered him until police arrived.

Carney told reporters that Yao’s episode was unexpected.

“This is the type of episode of an illness that can’t be predicted. It’s totally unexpected to everyone, most especially his parents,” Carney said.

Defense seeking hospitalization

The aftermath of the crime has been distressing for the defendant and his family, Carney said after the arraignment.

“Yao is in shock, trying to make sense of the situation,” said Carney.

“This whole tragedy is every parent’s nightmare; it’s a nightmare for the victim’s family,” he added.

Yao’s parents feel devastated about Stryker’s death, he said.

Carney said he hopes for an outcome in which Yao will be committed to a maximum security mental hospital, “where he will remain for the rest of his life,” he said.

Yao on probation following earlier crime

According to court records, Yao was supposed to be undergoing court-ordered mental health treatment at the time of the stabbing. The treatment was a condition of Yao’s pretrial probation put in place after Yao allegedly tried to break into a neighbor’s house.

Shortly before 4 a.m. on Sept. 9, 2017, Yao’s neighbors awoke to loud bangs that shook the house, they told police. According to the criminal complaint, one of the home’s residents observed Yao trying to break in through the sliding door in back. Yao was slamming the glass door with his arm and his body, the complaint states, and police found a broken clay plant pot on the deck outside the door, and dirt and clay on the door suggesting Yao had tried to throw the pot through the glass door, as well.

The incident left the residents visibly shaking and initially unable to speak clearly, the criminal complaint states. The complaint quotes one resident calling Yao “a total loose cannon.”

In interviews with the Star, many residents in Yao’s neighborhood described many run-ins with Yao over the years, and said many were afraid even when walking the short distance from their cars to their front doors at night. Winchester Chief of Police Peter MacDonnell confirmed the WPD had numerous encounters with Yao in the past years.

A Winchester police officer located Yao around 4 a.m. the night of the incident and placed him under arrest without incident. While being arrested, the criminal complaint states, Yao “appeared to be very disoriented and continued making comments about having voices in his head.”

At a Dec. 22 pretrial hearing, Yao was placed on one-year pretrial probation. A condition of his probation was that he continue unspecified mental health treatment, and take medications as prescribed.

Staff writer James Sanna contributed to this report