Gilroy Garlic Festival shooting: Suspect Santino William Legan, 19, identified as gunman who allegedly killed 3 people

Authorities are investigating the social media posts made by the suspected gunman in a California festival shooting that killed three people and injured more than a dozen others in an effort to determine a motive in the attack.

Gunshots rang out Sunday just before 6 p.m. local time at the popular Gilroy Garlic Festival in the city of Gilroy, some 80 miles southeast of San Francisco.

Santino William Legan, 19, was identified as the alleged shooter on Monday, law enforcement sources confirmed to ABC News.

Investigators are aware of recent posts Legan wrote on Instagram that include white supremacist themes, multiple law enforcement officials briefed on the probe told ABC News. One entry, which criticizes the garlic festival, appears to have been posted Sunday from the event, showing crowds of attendees.

Another message posted Sunday criticizes people of mixed racial heritage and makes reference to a book popular among white supremacists, officials said.

Legan's Instagram account has since been taken down.

Although officials are investigating the social media posts, they do not believe the posts prove that Legan's motive was an act of domestic terrorism.

Authorities responded to the home of Legan's father in Gilroy in the hours after the shooting, sources said. His family is cooperating with investigators, according to officials.

An assault-type rifle legally purchased in Nevada on July 9 and believed to have been bought by Legan was used in the attack, authorities said.

A law enforcement official briefing on the probe told ABC News the gun shop is Big Mike's Gun and Ammo in Fallon, Nevada.

"I did not know this individual. 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," the gun shop said in a statement posted on their Facebook page.

The motive is not known and no other accomplices are suspected of being involved in the shooting, officials said Monday.

A 6-year-old boy was among those killed.

"He was a really happy, loving boy," Maribel Romero, the boy's grandmother, told ABC San Francisco station KGO. "I want justice for my grandson."

A 13-year-old girl and a man in his 20s were also killed, Gilroy Police Chief Scott Smithee said in a news conference Monday.

Smithee told reporters at a late-night press conference Sunday that officers engaged the suspected gunman within a minute of the shooting and killed him.

Santa Clara Valley Medical Center and St. Louise Regional Hospital received a total of 11 patients with gunshot wounds from the shooting, according to Joy Alexiou, a public information officer at Santa Clara Valley Health System, which runs both hospitals.

One of the patients died, while three others were treated and discharged. The conditions of the remaining patients with gunshot wounds vary from fair to serious to critical, with some undergoing surgery. Eight other patients were admitted and treated for non-gunshot injuries, Alexiou told ABC News.

Candace Marquez and Cheryl Low, who were working at a festival booth, said they heard a "pop" and saw a man with a gun walking toward the tent.

"His gun was pointed to the ground, he was trying to put another clip in it and then he started walking to the left, away from us, and he started shooting," Marquez told ABC News chief anchor George Stephanopoulos in an interview Monday on "Good Morning America."

Marquez said her niece was at the tent and stayed with her boss' 3-year-old son.

"My niece was actually a hero," Marquez said. "She grabbed him and hid him under the table so he didn't get shot."

Low told ABC News that her boss and husband were both shot and taken to the hospital.

"One's in stable and one's in critical condition," Low said. "It's touch and go right now."

Marquez and Low told ABC News that the gunman, who appeared to be alone, did not talk and seemed to shoot at random.

"The next thing I saw, police had already shot and killed him," Low said.

Steve Janisch, a chef at the festival, said he was cleaning up and getting ready to leave when the gunshots started.

"I heard the shots, and got as many [of] as my guys, and got behind shelter in the alley," Janisch told ABC News. "I saw many people running in fear. Once we rounded everybody up out in the parking lot, we headed out per the direction of the authorities. I drove a mother and her two daughters safely to their car."

ABC News' Raymond Cinemato, Will Gretsky, Marilyn Heck, Ahmad Hemingway, Josh Margolin and Tracey Watkowski contributed to this report.