The gun store owner who claims to have sold the rifle used in Sunday's deadly shooting at a California food festival says he wants the 19-year-old assailant to “rot in hell.”

The comments on the Facebook page of Big Mike’s Gun and Ammo, based in Nevada, come as police continue to comb Santino William Legan’s scant social media presence for any clues that might reveal a motive in the attack. The Associated Press reported that investigators noted an Instagram account in Legan’s name posted twice in the minutes before he sprayed bullets into the Gilroy Garlic Festival crowd, wounding at least 12 and killing three, including a 6-year-old boy.

“The shooter in CA, I hope you rot in hell,” the post on Big Mike’s Gun and Ammo's Facebook page stated. “We pray for the victims. My heart hurts for all of them.”

In this aerial photo, emergency personnel, right, walk among vendor booths at Christmas Hill Park on Monday in Gilroy, Calif., the site of a shooting the day before at the Gilroy Garlic Festival. (AP)

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An earlier post signed by “Mike” described the shop as “a small home business” that sells to people “who we think are upstanding citizens to promote safe sport shooting."

“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,” the post stated. “I would never ever sell any firearm to anyone who acted wrong or looks associated with any bad group like white power. Everyone is my brother and sister and I am mourning for the families.”

Police on Monday said Legan legally purchased the rifle on July 9 in Nevada, where buyers must only be 18 years old. In California, the minimum age is 21.

A federal law enforcement official that spoke to the San Francisco Chronicle on condition of anonymity confirmed the rifle Legan used -- a WASR-10 semiautomatic -- was bought from Big Mike’s Gun and Ammo. But the official added that Legan broke the law by transporting the weapon to California, where the firearm violated the state’s assault weapons ban.

A police officer photographs a car outside the family home of Gilroy Garlic Festival gunman Santino William Legan on Monday. (AP)

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Police have not yet determined a motive for the shooting.

The Instagram account with Legan’s name on it identified himself as having Italian and Iranian descent. It only had three posts and one follower before the social media site took it down Monday. The first post on it was made Thursday, and the final two appear to have been made just minutes before the shooting.

One of those two posts shows a picture of Smokey the Bear and urges people to read a 19th-century text that the Associated Press reports is favored by some fringe groups.

“Why overcrowd towns and pave more open space to make room for hordes of [Latinos] and Silicon Valley white t---s?” the post adds.

Another message shows an image of what appears to be people streaming into the food festival.

“Ayyy garlic festival time, come get wasted on overpriced s—t” that post says.

Susan Meyers and husband Michael Oshan listen to a hymn during a vigil Monday for victims of the shooting. (AP)

Investigators also carried out search warrants Monday at properties connected to Legan and were seen at one of the houses carrying out what appeared to be bags of evidence. The vehicle Legan allegedly drove to the festival is being searched, too. Police have not yet revealed what's been found.

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A resident who lived across the street from one of the dwellings – the Legan family home in Gilroy – told the Associated Press that Santino came from “a nice, normal family,” but has not lived at the house during the past year.

"How do you cope with this?” said Jan Dickson, the neighbor. “They have to deal with the fact that their son did this terrible thing and that he died.”