GILROY, Calif. — The gunman who killed three people and wounded a dozen more at the Gilroy Garlic Festival in Northern California was an angry 19-year-old who had recently waded into the world of white supremacy.

Santino William Legan, who was shot dead by police Sunday before he could do more damage, posted online about an 1890 racist manifesto, “Might is Right or The Survival of the Fittest,” NBC News confirmed.

“Read Might is Right by Ragnar Redbeard,” Legan posted on his Instagram page. He then used a slur against mixed-race people and complained about “hordes” of them “overcrowding” towns.

Redbeard, which was a pseudonym, argued that only strength and violence determined what is morally right. The work, which is filled with misogynistic and anti-Semitic rhetoric, is a staple among neo-Nazis and white supremacists on extremist sites.

And the phrase “might is right” is often posted as a sort of motto or catchphrase indicating white supremacy on neo-Nazi extremist forums.

Legan was also apparently no fan of the festival, a three-day food fair that began in 1979 to celebrate the local garlic industry — and which was in walking distance from his home on a tree-lined street in Gilroy.

“Ayyy garlic festival time,” his post read. “Come get wasted on overpriced s—.”

Below that was a post from someone named futboieden, which read “when you get too wasted and accidentally shoot up the festival.”

Just who futboieden was remained unclear a day after the nation was left grappling with yet another mass shooting. This one claimed the lives of 6-year-old Stephen Romero, a 13-year-old girl, and a man in his 20s. The names of the other two victims were not released.

Investigators also said they were looking into reports that Legan might have had an accomplice but said that had not been confirmed. They said Legan legally purchased the AK-47-style assault rifle that he used in the shooting on July 9 in Nevada.

“We don’t have a motive for the shooting as yet,” Gilroy Police Chief Scot Smithee said during a press conference.

Legan was from a family of boxers. He was coached, along with his brothers, by their father, Tom.

Neighbors said the family converted its garage into a boxing gym and the boys were often seen sparring with each other.

“I’m so confused and hurt for the parents to go through this,” said Elia Scettrini, 65, who lives two doors away and teaches Spanish at Gilroy High School.

Scettrini said Legan just graduated from high school and described the family as friendly and polite.

Police on Monday could be seen carrying several bags of evidence out of the family home and searching a car parked outside.

This was not the first time the Legan family has found itself in the crosshairs of a police investigation.

Legan’s grandfather Thomas Legan was a Santa Clara County supervisor running for re-election in 1988 when he was accused of molesting one of his daughters six years earlier.

Thomas Legan, who died last year, insisted he was innocent and maintained his ex-wife had manipulated the girl into making a false accusation. A jury agreed and found him not guilty after six days of deliberations.

The Gilroy Garlic Festival is one of the county’s best-known food festivals and has been held for 41 years. It draws hundreds of thousands of paying visitors every year.

Rep. Dan Lipinski, D-Ill., said he and his wife, Judy, were in the crowd Sunday when the shots rang out.

“The shooter was not far from us as we heard the loud ‘pops,’ which seemed to get closer as we ran,” the congressman said in a statement.

Ingram reported from Gilroy, and Zadrozny and Siemaszko from New York.

Ben Collins contributed.