While a motive remains unclear in the deadly rampage at the Gilroy Garlic Festival, authorities have found an array of evidence suggesting the deceased killer was gearing up for warfare and survival, seizing not only high-powered guns but tactical gear, a personal fire shelter and a clown mask.

According to a search warrant released Thursday, police recovered an AR-15-style assault rifle in a camouflage bag along with three high-capacity ammunition magazines from the family home of gunman Santino Legan, along with a trove of survival gear and an open bottle of Jack Daniel’s whiskey from his vehicle, which was parked outside the festival.

Gilroy police served the warrants the day after the 19-year-old man opened fire on crowds at the event on July 28, killing two children and one man and wounding 13 others with an AK-47-style assault rifle. Authorities have said Legan purchased at least two of his weapons legally in Nevada, where he had moved recently, before illegally bringing them into California.

Police opened fire on Legan as he sprayed gunfire at the unsuspecting festivalgoers before the killer fatally shot himself in the head a minute after unleashing his barrage.

Inside Legan’s family home on Churchill Place in Gilroy, the new documents show, police collected numerous items, including a “Franklin Armory AR-15 assault rifle” that was in a camouflage bag against a wall in the master bedroom. The bag contained three “high-capacity magazines with .223 ammunition.” Two magazines had 29 rounds, and another had 31, police said. It was not clear if the bag belonged to Legan.

California law prohibits the sale of assault weapons that meet certain criteria and the sale of ammunition magazines that hold more than 10 rounds. In addition, California residents must be at least 21 years old to buy guns. But Nevada and federal law contain looser regulations, differences that California Attorney General Xavier Becerra lambasted after the Gilroy shooting.

In Legan’s 2011 Silver Honda Accord that was left at the scene of the shooting, police recovered 53 items, including a manual on how to load a 75-round drum ammunition magazine, a clown mask, four other masks, a Remington 870 tactical shotgun, a tactical harness, a personal fire shelter, binoculars, a wilderness training manual, a signal horn, ear plugs, bottle rockets, a pellet gun and a library card from Mineral County in Nevada, where the killer recently moved.

The document was released as the South Bay community continued to reel from the tragedy and authorities worked to uncover a motive — and whether anyone assisted Legan or knew about his plans in advance. The FBI said Tuesday it opened a domestic terrorism investigation into the shooting, giving the inquiry higher priority and more resources.

Legan fired 39 rounds during his rampage after cutting through a fence and storming into the festival wearing a bullet-resistant vest, officials said. Police said he brought a 75-round drum magazine with him that still contained 71 rounds. Legan also had four 40-round magazines and shotgun ammunition.

Investigators last week searched Legan’s apartment in Nevada where they found more items, including a gas mask, bullet-resistant vest and empty boxes of ammunition, along with unspecified reading material on radical ideologies.

Both the WASR-10 semiautomatic rifle — the AK-47-style weapon used in the attack — and the shotgun found in the car were purchased by Legan in Nevada, police said.

Before his attack, Legan posted cryptic messages on his Instagram account, criticizing demographic changes in his community and touting a 19th century book, “Might Is Right,” that has seen a revival in white supremacist circles. The FBI said it has found evidence he was “exploring varied and competing violent ideologies,” but officials said it was unclear which, if any, he may have settled on.

His victims were two children from the South Bay, 6-year-old Stephen Romero and 13-year-old Keyla Salazar, and Trevor Irby, 25, who recently moved to Santa Cruz from upstate New York. Nearly 200 people attended Keyla’s funeral Tuesday in San Jose.

The shooting was the first of three mass killings across the country in which rifle-wielding white gunmen killed large numbers of strangers. On Saturday morning, a gunman killed 22 people at a Walmart in El Paso, Texas. About 12 hours later, a gunman killed nine people in a popular area for nightlife in Dayton, Ohio.

Evan Sernoffsky is a San Francisco Chronicle staff writer. Email: esernoffsky@sfchronicle.com Twitter: @EvanSernoffsky