U.S. Attorney Nealy Cox credits Williams' grandmother for probably saving the lives of strangers.
"This was a tragedy averted," Cox said in a statement. "I want to praise the defendant's grandmother, who saved lives by interrupting this plot, as well as the Lubbock police officers and federal agents who investigated his unlawful acquisition of a deadly weapon."
When agents searched Williams' hotel room, they found the AK-47 and 17 loaded magazines and multiple knives laid out on the bed, Cox said in the release.
He also had black tactical gloves with fingers cut off, a black trench coat, black tactical pants and a t-shirt that read "Let 'Em Come," according to the authorities.
Williams purchased the gun on July 11, but the ATF said it received the form on July 21 and Williams had lied about his address on the form. He had been kicked out of the home of relatives and had been staying with a roommate at an address that was different from what his license showed, federal authorities said.
Williams was charged with making false statements to a firearms dealer, CNN reported.