WINCHESTER — On the narrow street where Jeffrey Yao lived, residents said they lived in fear.
Once a quiet youngster, the 23-year-old Yao had transformed into a terror, they said, trying to break into homes, smashing glass in the road, shattering his own windows, and making threatening gestures at passersby. Terrified neighbors said they had reported Yao to authorities.
On Saturday morning, the violence that Yao’s neighbors feared allegedly erupted in the reading room of the Winchester Public Library.
Yao strode into the granite building with a 10-inch hunting knife, walked up behind a young woman who was reading at a table, and stabbed her to death, according to Middlesex District Attorney Marian T. Ryan.
The 22-year-old woman, who was not identified, fled into the library’s main lobby, but didn’t survive the slash and stab wounds to her head and upper torso, said Ryan, who spoke at a news conference at the Winchester police station.
At least two patrons rushed to her aid, including a 77-year-old man who was stabbed in the arm during the confrontation, while others corralled Yao until police arrived, she said. The wounded man is expected to recover.
Hours after the deadly attack, Yao’s Farrow Street neighbors stood in groups, trying to process the tragedy they said they had all seen coming.
“We knew something would happen,” said Leslie Luongo, who lives nearby. “We didn’t know when.”
Some Winchester residents said the attack had an echo of the Florida school shooting in which a teenager who was well known to authorities allegedly killed 17 people.
“It’s very unsettling especially with everything else that’s going on,” said Irina Puscasu, a Winchester resident who was headed to the library when she saw an ambulance make a U-turn in front of the main entrance. A police officer was turning people away at the door.
“It’s a place we take our families. I would never have thought of not feeling safe in a library,” she said. “The library is such a peaceful place.”
Russ Ganz said he was walking past the library at about 11 a.m., and saw police cars and a woman, who appeared to have injuries to her shoulder, being taken out on a stretcher. She was upright and conscious, he said.
“I thought somebody fell down the stairs,” he said.
Yao is being held without bail pending his arraignment, scheduled for Monday in Woburn District Court, where he will face charges of murder and assault with attempt to murder, Ryan said. She said Yao was known to police but didn’t provide details.
Luongo said she was so afraid of Yao that she would run to her car every day when she left the house at 5 a.m. to go to work. She said residents kept their children indoors, kept baseball bats nearby, and locked their doors.
One neighbor, who spoke on condition of anonymity to protect his family’s privacy, said that late last summer, he and his wife had awoken to hear Yao slamming against their front door.
Before he even looked outside, the neighbor said, he knew it was Yao. The young man had gone from making an obscene gesture at his pregnant wife as she walked down the street a few years earlier to hurling an Adirondack chair at another neighbor’s door, he said. The police were summoned, and Yao was arrested.
A few months later, he said, a court official called him about the case. The neighbor said he told the official then that Yao was a danger, saying: “He is going to kill somebody.”
“We warned them,” the neighbor said. “We warned them — I can’t believe this. The only one that’s going to protect you in this world is yourself.”
Luongo said she, too, had gone to authorities with her fears late last summer, heading to the Winchester Police Department to tell them she and other neighbors were afraid.
Police told her not to worry, she said: They were shadowing Yao when he went out at night.
“They knew he was on their watch list,” she said.
Winchester police declined to comment on her allegation Saturday.
Alex McDonough, 24, who went to Winchester High School with Yao, said his former classmate began exhibiting strange behavior during his junior or senior year. He boasted about not bathing and posted an insensitive comment on Facebook about a student who had died, McDonough said.
“He started getting strange and started alienating people,” said McDonough, who lives in Providence. “No one’s quite sure why it happened or how it happened.”
He said he tried to talk to Yao about his hygiene, but Yao didn’t seem interested. He eventually blocked Yao on social media and lost track of him after graduation in 2012. But he said he didn’t know Yao to be violent.
“It was kind of tough to watch the gradual decline,” McDonough said.
No one answered the door Saturday at Yao’s home. Neighbors said his parents were good people who were doing the best they could to help their son, who had wrestled and ran on the cross-country team in high school.
“He was a wonderful kid,” said one woman, who declined to give her name. Her sister went to school with Yao, she said, and her son wrestled with him. Yao had always seemed young for his age, and small — never a threat.
“Something bad happened in his head,” she said.
Stephanie Ebbert of the Globe Staff contributed to this report. Evan Allen can be reached at email@example.com. Follow her on Twitter @evanmallen.
Laura Crimaldi can be reached at firstname.lastname@example.org. Follow her on Twitter @lauracrimaldi.