Officials confirm to ABC News this image taken from Instagram shows Santino William Legan, who's accused in the deadly shooting in Gilroy, Calif. on July 29, 2019.

"Kind of a loner"

Instagram posts before the shooting

EMBED >More News Videos The gunman in the California festival shooting appears to have posted two photos on Instagram that day, including one minutes before he opened fire. In the caption, he said to read "Might is Right," a book published in the 1800s. The misogynist and anti-Semitic work is used by neo-Nazis and white supremacists on extremist sites.

Motive is still a mystery

Suspect purchased the gun legally

Suspect grew up in Gilroy area

Suspect's family was well-known for different reasons

Family of suspect initially was worried that he was a victim of the attack

Neighbors surprised by the suspect's involvement

GILROY, Calif. -- The gunman in the Gilroy Garlic Festival shooting was identified by police as 19-year-old Santino William Legan, but why he opened fired still remains a mystery. Police say Legan killed three three people - a 6-year-old, a 13-year-old and a 25-year-old -- at the annual California event. Here's what we've learned about him so far.The 19-year-old gunman who opened fire at a Northern California food festival was "kind of a loner" and much of his life was shrouded in mystery, the FBI said as investigators searched for a motive."We understand him to be kind of a loner," said Craig Fair, deputy special agent in charge of the FBI's San Francisco Division. "People who act alone are exceptionally dangerous because they ... may not communicate their plans, intentions, mindset - they may not impart that on other people."On the day of the attack, Legan urged his Instagram followers to read a 19th century book popular with white supremacists on extremist websites. He also complained about overcrowding towns and paving open space to make room for "hordes" of Latinos and Silicon Valley whites.Legan posted a photo from the festival minutes before opening fire, saying, "Come get wasted on overpriced" items. His since-deleted Instagram account says he is Italian and Iranian."Might is Right," which was first published in the 1890s, is popular among white supremacists and viewed by some as a movement manifesto. It's available online on sites like Amazon. Chapman University Sociology Professor Peter Simi says it's popular with some far right groups."It's long been a staple within the white supremacist movement," he explained. "It's part of a much larger set of propaganda literature that folks who identify with different aspects of white supremacist ideology will use as a way to reinforce and support their beliefs."The book lays out an amoral philosophy of survival of the fittest, arguing that war and violence are necessary to the natural order."This notion of survival of the fittest, some individuals, some groups are essentially more fit for survival," Simi said. "They're biologically or culturally superior to other groups in their view."The professor adds that " Might is Right" shares themes with similar books that celebrate lone-wolf style heroes. A vision that can draw in loners, and provide them with a dangerous sense of validation."And so, for instance, white supremacists have promoted that lone individuals take essentially the law into their own hands and act violently," Simi said. "And they've been promoting that strategy for decades."Multiple officials tell ABC News that investigators are aware of the recent Instagram posts and that those posts are now part of the active investigation.The law enforcement officials caution that while they are investigating the social media posts, they do not believe the posts prove Legan's motive was domestic terrorism. Though the authorities said they do not yet know the motive for the massacre, they believe it could turn out that Legan was simply another angry and unstable man who acted out violently and used racial and ethnic anger to justify the shooting he was contemplating.While some killers in recent mass shootings have taken to posting writing or even broadcasting their attacks, there are cases where shooters leave very little public footprint and no motive is uncovered.That includes the gunman who carried out the deadliest mass shooting in modern U.S. history. No clear motive was found for why Stephen Paddock killed 58 people at an outdoor music festival in Las Vegas in 2017, though the FBI said he sought notoriety.Gilroy Police Chief Scot Smithee told reporters that authorities believe Legan acted alone but are still investigating. Investigators tracked Legan's movements around town before the shooting, and video shows him visiting stores alone, Smithee said.Officials say Legan's family is cooperating with investigators.Authorities say Legan was recently living in Nevada, where he purchased two guns - the AK-47-style semi-automatic rifle he used in the attack and a shotgun that was found in his car near the festival.A law enforcement official briefed on the probe tells ABC News the gun shop that sold the suspect the gun was is Big Mike's Gun and Ammo in Fallon, Nevada.The shop released the following statement:Legan briefly lived in Nevada, according to law enforcement, where he lived in the small community of Walker Lake in a triplex.Legan was living in a triplex in the small community of Walker Lake, a remote northern Nevada community, and had not had any run-ins with the law, officials said."He appears to have moved into Mineral County this spring and maintained a low profile," Sheriff Randy Adams said in a statement.Officials searched the apartment, seizing empty shotgun and rifle boxes, a gas mask, empty ammunition boxes, electronic devices, pamphlets on guns and a sack full of ammunition casings, prosecutors said.One neighbor remembers seeing him around."We used to see him walk up and down the road to the mailbox and walk down to the lake, stuff like that. Never got - never said hi, well might have said hi to him, but never got to know him or anything like that," said Marty Neff.ABC7 News obtained video of Legan graduating from Gilroy high school. He attended Monte Vista Christian School in Watsonville for his freshman through Junior years of high school before transferring to Gilroy. Monte Vista Christian School released a statement on Monday:Legan's grandfather, Tom Legan Sr., was a two-term Santa Clara County supervisor who passed away in 2018. Press reports from 1988 show that he was tried and acquitted of molesting his teenage daughter. The grandfather maintained that his ex-wife manipulated their daughter into raising the charges as part of a vindictive plot to extract more financial support, according to the Associated Press report from the trial. He lost re-election four days after being acquitted.His son and the suspect's father Tom Legan Jr. was a competitive runner who set a world-record as a senior runner. He ran with the Bay Area based Strawberry Canyon Track Club.The suspect's brother, Rosino Legan, was an acclaimed amateur boxer. According to an article in the Gilroy Dispatch dated July 2017, Rosino is training for the 2020 Olympics coached by his father.The paper reported that the suspect would even spar with his brother.Kawika Palacios a family friend of the Legan's told ABC News that Legan and his brother were very active and would train with each other in the garage."They train, they run, they're really active," said Palacios. "The conversations I had with him, I would never suspect something like this so, it's kind of a shocker."In Gilroy, outside the Legan family home a man identifying himself as the Legan family trainer says his heart breaks for the victims and also for the Legan family who he says he loves like his own.Members of Legan's family were initially worried that the 19-year-old was a victim of the attack. A family friend tells ABC News Legan's brother was looking for him after the shooting, not knowing he was the gunman.Kyle Dickson, who lives across the street from the family's Gilroy home, called the family upstanding."(A) good family lived here for a long time," said Dickson. "I know the parents. They're very nice, always friendly. I talk to them every time I see them. Just a great family."According to Dickson, the SWAT team, police and ATF agents descended on the home on Churchill Place around midnight. He says only a woman was home.Police stayed at the home for about eight hours. They left at 8:15 a.m. with bags full of possible evidence.