By Susan Scutti, CNN

(CNN) -- Authorities say Santino William Legan killed three people, including a 6-year-old boy and a 13-year-old girl, at the Gilroy Garlic Festival with a rifle he legally purchased three weeks ago in Nevada.

The third fatality was a man in his 20s, and at least 12 other people -- ranging in age from 12 to 69 years old -- were injured in the attack, authorities said in a news conference Monday.

The gunman, 19, opened fire on the crowds attending the outdoor festival around 5:40 p.m. local time Sunday, according to Gilroy Police Chief Scot Smithee, and was fatally shot within one minute by three officers already on the scene.

"It could have gone so much worse, so fast," said Smithee, who thanked the officers on duty at the festival for responding so quickly.

Monday morning, several police cars were outside the home of Legan's father in Gilroy, a city of about 48,000 people.

What officials say about the shooter

The shooter had an assault-style rifle and appeared to shoot at random, Smithee said. The SKS, an "AK-47 type" rifle, was purchased legally in Nevada on July 9, he said, adding that his understanding is that Legan purchased the weapon himself.

It is believed that Legan, though originally from Gilroy, had been living with family members in Nevada, Smithee said, adding that he does not know how long Legan had been back in California.

Witnesses reported seeing a second individual who has not yet been identified or found.

"We've had multiple reports but different people give different versions so we don't know at this point" if another suspect was involved or to what extent, said Smithee.

Police believe the suspect cut through a fence near the creek with a tool to enter onto the event grounds in Gilroy, which is about 30 miles south of San Jose. There were no posted security cameras at the festival, according to Smithee.

The FBI Evidence Response Team from San Francisco has arrived and is currently processing the scene at the festival, according to the official source. The Bureau of Alcohol, Tobacco, Firearms and Explosives is also there, along with various state and local police and fire departments, though no further information is available.

Instagram posts hours before attack

An Instagram account, which was created four days ago under the suspect's name, posted two messages shortly before the attack began.

One included a photograph of people walking around the Garlic Festival with the words "Ayyy garlic festival time Come get wasted on overpriced sh."

The other, which included a photo of Smokey the Bear holding a sign saying "Fire Danger High Today," stated: "Read Might is Right by Ragnar Redbeard. Why overcrowd towns and pave more open space to make room for hordes of mestizos and Silicon Valley white tws?"

A mestizo is a person of mixed descent, commonly white and Hispanic or white and American Indian.

"Right is Might," first published in the late 1800s, has been described as a white supremacist text that promotes anarchy while vilifying Christianity. The book calls Jesus the "true Prince of Evil" and says that the natural order is a world at war in which the strong must vanquish the weak and white men must rule over those of color.

Eyewitness description

Julissa Contreras, who was standing within 20 feet of the shooter, told CNN's Natalie Allen the gunman "looked like the average guy.

"White, about in his 30s, under 6 feet. He was wearing tactical gear, had a baseball cap on, sunglasses, and he had a semi-automatic rifle. He was dressed for what he was there to do."

Contreras also said that "he didn't look angry," though she added that his "expression wasn't really something I could read on his face but he seemed very well-versed in what he was doing... and he seemed confident in his use of the gun."

Members of the band Tin Man, who were starting an encore when the shooting happened, told CNN the gunman was carrying a rifle and wearing a green shirt with a gray handkerchief tied around his neck. They also said that he sprayed gunfire from side to side with no direct target, creating pandemonium among the crowd.

Recollections of Legan

Josh Carman, who was a senior at Gilroy High School when Legan was a sophomore, told CNN that he knew who Santino Legan was but didn't know him well.

"I never had any classes with him or anything, so we never really got acquainted, I just have definitely seen him around school," he said. "He wasn't the type of person that everyone knew, but he didn't stand out as being too abnormal."

Tanner Eastus, another Gilroy High School student who will enter senior year this fall, shared Carman's opinion. Eastus, who was at the festival with friends when the shooting happened, told CNN that "[Legan] seemed like a normal guy. I don't believe anyone expected anyone to do something like this, especially in Gilroy."

Rosana Mendoza, a neighbor, recognized a photo of Legan that CNN showed her.

Last night at about 11:30 p.m., multiple law enforcement vehicles went to the house of the Legan family and she talked briefly with authorities, she told CNN.

"They told me the suspect is three houses from our house. I said, what, this teenager? They are all very nice."

She said she does not know the family well but she would see the parents walking or Legan and his brothers playing basketball or hanging out in the front yard.

"I would say hi to them. They are very nice kids. When I found out this morning, I was shocked. This is a quiet place."

What is not known

Police said a motive for the shooting remains unknown.

"It seems this was a random act, but we've got a long way to go before we can determine what his motivations were," said Smithee.

Craig Fair, a deputy special agent in the San Francisco division of the FBI who is assisting in the investigation, said the primary quest at the moment is to understand Legan's motivation, ideological leanings and affiliations. As 30 members of the FBI's evidence recovery team comb the scene, nothing has been determined and nothing has been ruled out at this point, said Fair.

Josh Campbell, a former FBI supervisory special agent, told CNN's Jim Sciutto that witnesses provided information to the police about a possible second suspect but "we know there's almost always never a second shooter" in these types of mass killings.

Campbell noted the shooter was prepared, as demonstrated by the fact that he cut through the fence to avoid security and metal detectors. Among the questions that need to be answered is why the shooter wanted to get into the annual food festival, Campbell said, adding that the answer would speak to the gunman's motive.