Corrections and Clarifications: Corrects the title of a book that gunman, identified by authorities as Santino William Legan, was reported to have urged people to read in a social media post before the shootings.

GILROY, Calif. — The 19-year-old who police say turned an assault weapon on festival-goers here was known as a quiet teen from an athletic family who stayed out of trouble, neighbors said Monday.

But there clearly was another side to Santino William Legan, one that motivated him to dress in fatigues, cut his way through a fence and, police say, to kill three people, including a 13-year-old girl and 6-year-old boy, and wound at least a dozen others at the Gilroy Garlic Festival.

It was that side that appears to mirror the since-deleted Instagram account in Legan's name. The postings, which included a photo of the festival, urged people to read a book that argues power alone can establish moral right. The book, "Might Is Right," was published in the late 1800s.

And, in a posting just before the attack, the Legan account said: “Ayyy garlic festival time” and “come get wasted on overpriced (stuff).”

When asked whether the gunman had any prior run-ins with the police, Gilroy Police Chief Scot Smithee said only that detectives are investigating Legan's background. His weapon,identified by federal authorities as a variant of an AK-47 assault rifle, was purchased in Nevada, where Legan apparently had an apartment. Legan is originally from Gilroy, the chief said.

A gun seller in Fallon, Nevada, posted a note apologizing to families for having sold the weapon to Legan.

"I did not know this individual," said the posting on the Big Mikes Gun and Ammo page on Facebook. "He ordered the rifle off my internet page. When I did see him, he was acting happy and showed no reasons for concern. I would never ever sell any firearm to anyone who acted wrong or looks associated with any bad group like white power. Everyone is my brother and sister and I am mourning for the families."

The post was signed: "Mike."

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Those who knew Legan's family say they would've had no suspicion a mass shooter had been in their midst until police came banging on the door of the family home at 11 p.m. Sunday. Hours earlier, police say, Legan had attacked the three-day festival that attracted up to 100,000.

"My dad was in the backyard feeding the cats when he heard 'Gilroy Police Department, open up!' " coming from the Legan family home, said Andrew Sanchez, 19, who lives close enough to see into the Legan family's yard.

Legan and his two brothers hailed from a family known for its love of athletics. The family was so involved in boxing they had a ring set up in the garage of their two-story home, said neighbor Elia Scettrini, 65, a retired Gilroy High School teacher who lives two doors away in a quiet neighborhood about four miles from the shooting scene.

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She said she'd see Santino and his brothers biking or jogging around the neighborhood.

Legan was not a troublemaker in high school, where she taught literature and Spanish until 2017, though he was never in one of her classes. "At school, you know who (are) the bad kids," said Scettrini, and he was not one of them.

The star boxer in the Legan family was Santino's older brother, Rosino. Neighbor Larry Soto, 59, said he had donated to help the brother's boxing career.

Rosino Legan, is a decorated amateur boxer, who at one point was ranked second in the nation in his weight class, according to the Los Angeles Times. As recently as 2017, the Gilroy Dispatch described Santino Legan as one of his older brother's sparring partners.

Rosino Legan's trainer, Jerome Turcan, has worked with the eldest Legan brother since he was 7, and he's also a friend of the brothers' father, Tom Legan. He said he didn't know Santino Legan outside of seeing him a few times when he was a kid.

The family is in great pain right now, Turcan said in a text message.

"They are good people and they are devastated," Turcan said. "That's all I know."

What we know about the shooting:The gun was purchased legally. The cops had body cameras

Not everyone in the community knew the Legan family. Bobby Holmes, 44, is the general manager of two 9Round Fitness franchises — one in Gilroy and another in neighboring Morgan Hill — but didn't know the Legans.

Nevertheless, Holmes said he and his family hunkered down inside last night, turned off the lights and locked their doors. He said that many other local families are currently doing the same thing, "just trying to figure this thing out mentally."

"Everybody knows everybody," Holmes said. "San Jose to Morgan Hill to Gilroy is a pretty tight-knit community."

Szydlowski reported from Gilroy. Weber and Woodyard reported from Los Angeles

Contributing: Associated Press