GILROY, Calif. — Before a 19-year-old gunman opened fire on a famed garlic festival in his California hometown, he urged his Instagram followers to read a 19th century book popular with white supremacists on extremist websites, but his motives for killing two children and another young man were still a mystery Monday.

Santino William Legan posted the caption about the book which claims race determines behavior. It appeared with a photo of Smokey the Bear in front of a “fire danger” sign and also complained about overcrowding towns and paving open space to make room for “hordes” of Latinos and Silicon Valley whites.

In his last Instagram post Sunday, Legan sent a photo from the Gilroy Garlic Festival. Minutes later, he shot into the crowd with an AK-47 style weapon, killing a 6-year-old boy, a 13-year-old girl and a man in his mid-20s. Under it, he wrote: “Ayyy garlic festival time” and “Come get wasted on overpriced” items. Legan’s since-deleted Instagram account says he is Italian and Iranian.

The postings are among the first details that have emerged about Legan since authorities say he appeared to fire at random. Police patrolling the event responded within a minute and killed Legan as he turned the weapon on them.

He legally purchased the semi-automatic assault rifle this month in Nevada, where his last address is listed, and the age limit for firearms purchases is 18.

Legan grew up less than a mile from the park where the city known as the “Garlic Capital of the World” has held its three-day festival for four decades, attracting more than 100,000 people with music, food booths and cooking classes.

Authorities were looking for clues, including on social media, as to why the son of a prominent local family went on a rampage. His father was a competitive runner and coach, a brother was an accomplished young boxer, and his grandfather had been a supervisor in Santa Clara County.

Police said they don’t know if people were targeted, but it appears he shot indiscriminately. Twelve people were injured.

Police searched Legan’s vehicle and the two-story Legan family home and an apartment they believed Legan used. Officials didn’t say what they found.