Stephen Paddock bought nearly three dozen guns in the year before he unleashed carnage on a crowd in Las Vegas, killing 58 and wounding more than 500.
The high-stakes gambler purchased the 33 firearms _ most of them rifles _ since October 2016, officials with the Bureau of Alcohol, Tobacco and Firearms told CBS News on Wednesday.
Jill Snyder, special agent in charge for the agency, said the ATF wouldn't be tipped off by the high volume of sales unless two or more handguns were sold at one time.
"We wouldn't get notified of the purchase of the rifles," Snyder said on "CBS This Morning." "There's no federal law requiring that."
Paddock might have bought guns as recently as Thursday, three days before he fired hundreds of bullets on the Route 91 Harvest festival.
He fired the rounds from his 32nd-floor hotel room at Mandalay Bay Casino and Resort, where authorities found 23 firearms after the shooting.
A dozen of those guns were fitted with bump stocks, which allowed the gunman to fire rifles almost as if they were automatic _ unleashing mass amounts of mayhem in a short time.
The rifle magazines he used held anywhere from 60 to 100 bullets each, Snyder told CBS News, meaning he didn't often need to stop and reload.
Authorities are also investigating whether Paddock practiced at any of the gun ranges on or near the Las Vegas Strip.
When a licensed gun dealer sells two or more handguns to an unlicensed buyer within five straight business days, it has to be reported to the ATF.
The only states required to report transactions by licensed sellers involving multiple rifles sold within that time frame are Arizona, California, New Mexico and Texas. Those rules have been in place since 2011 _ enforced on rifles that shoot anything larger than a .22-caliber bullet _ are set to expire at the end of this November.
