A gunman who opened fire on festivalgoers, killing three people – including two children – and injuring at least 12 in northern California appeared to reference a white supremacist book on Instagram just hours before the tragedy.

Law enforcement officials in Gilroy said on Tuesday that they were not yet ready to comment on their early investigation into the shooter’s ideology or motivation. Different sources of information about the shooter “don’t all align”, the Gilroy police chief, Scot Smithee, said.

Sunday’s attack, in which the suspect started shooting, apparently indiscriminately, with an AK-47 style rifle at the Gilroy garlic festival, a popular food and music event about 70 miles south of San Francisco, has shocked the community.

The three young people killed, two of whom were children, have been named as Stephen Romero, six, Keyla Salazar, 13, both from San Jose, and Trevor Irby, 25, from upstate New York.

Investigators searching the 19-year-old suspect’s residence in Nevada found empty boxes of ammunition, a gas mask and a bulletproof vest, the San Francisco Chronicle reported. Smithee said he could not yet comment on whether the suspect – identified by police as Santino William Legan – had been planning any additional attacks.

An Instagram account bearing the name of the suspect published a picture before the shooting with a “fire danger high today” sign. A caption cited a text dating from 1890, alongside racist comments. The account was later taken down. Legan was shot dead by the police after opening fire at the festival.

“Read Might is Right by Ragnar Redbeard,” the gunman reportedly wrote. “Why overcrowd towns and pave more open space to make room for hordes of mestizos and Silicon Valley white twats?”

On Tuesday, the Gilroy police chief Smithee told reporters at a news conference that he could not yet confirm whether that Instagram account did belong to the shooter.

The FBI is continuing to investigate if the alleged shooter “was in line with any particular ideology”, Craig Fair, a deputy special agent in charge with the FBI in San Francisco, said on Tuesday, including by reviewing social media accounts, digital media, conducting interviews and investigating what thoughts and ideas the shooter may have shared with others. He also did not confirm whether the widely-publicized Instagram account was linked to the shooter.

“Everyone wants to know the answer: why?” the Gilroy police chief Smithee told the LA Times earlier. “If there’s any affiliation with other people, or groups of people, that could potentially pose a threat in the future, that all plays in.”

Police previously said they were searching for a second person who witnesses said could have been associated with the shooting, but Smithee said on Tuesday that “our investigation is leading us more and more to believe there was not a second person involved”. In video footage of the shooter captured by local stores, he was alone, the police chief said.

The gunman cut through a fence to avoid security at the festival, which attracts around 100,000 people who gather every summer to eat garlic-loaded food to celebrate the local crop, amid live music and family entertainment.

He started shooting the assault weapon close to an inflatable bouncy castle where children were playing.

It is illegal to buy or transport such a weapon in California, where gun laws are comparatively strict, but the state attorney general Xavier Becerra said it was legally purchased in Nevada.

A vigil for the victims of the Gilroy shooting. Photograph: Kenneth McCain/ZUMA Wire/REX/Shutterstock

He said: “There is a very strong likelihood as we develop the evidence that the perpetrator in this particular case violated California law on top of the crimes of homicide and so forth, the crimes that we have that are meant to prevent individuals from carrying out this type of activity.”

Hundreds gathered in Gilroy for a vigil on Monday night, where residents came together to mourn. A city council member led a call and response chance of “We are Gilroy strong”.

Mayor Roland Velasco told the crowd: “Maybe not today, but there will be a day when we start to heal, and the reason for that is we cannot let the bastard who did this to us tear us down.”

Katiuska Vargas, whose niece Salazar was killed in the attack, said the teenager may have inadvertently saved a life.

The family started running as shots rang out, but Salazar stayed with a wounded relative with mobility issues. “If Keyla hadn’t been there, her stepfather’s mother would have been shot,” Vargas said.

The State governor Gavin Newsom decried lack of federal legislation. He said: “This is America today – the shootings continue. Loved ones are buried. Children are gunned down. And Congress does nothing.”