One of many shocking aspects of the Las Vegas shooting on Sunday night—during which at least 59 people were killed and more than 500 injured—was the sheer size of the arsenal stockpiled by shooter Stephen Paddock.

After bursting into his room on the 32nd floor of the Mandalay Bay Hotel—where police said Paddock had killed himself—officers found 23 guns in the hotel room, including high-powered rifles capable of penetrating police armor.

When searching Paddock’s home in Mesquite, Nevada, police found a further 19 weapons, several thousand rounds of ammunition and explosives.

So how was Paddock able to not only amass such an armoury, but get it into a Las Vegas hotel room? 

Workers at several gun shops in Nevada have said that they sold weapons to Paddock in the past, but have maintained that the gunman passed all necessary background checks.

David Famiglietti of the New Frontier Armory told NBC News that Paddock bought a rifle and a shotgun in spring. The sale was handled by officials from the Bureau of Alcohol, Tobacco, Firearms and Explosives, and Famiglietti said that Paddock was not a regular customer at the store.

“We’re very sad about the news of this tragedy. We’re in the business of selling firearms legally and took all precautions on this sale, as we do with all sales,” he said.

The general manager of Guns & Guitars, a gun store in Mesquite, also told NBC News that Paddock had shopped there but did not specify what weapons he bought. “He never gave any indication or reason to believe he was unstable or unfit at any time,” said Christopher Sullivan in a statement.

Read more: Las Vegas shooting: Officers took more than an hour to storm gunman's room

But according to gun control lobbyists, the fact that Paddock passed background checks for purchasing the weapons is not saying much. Nevada has some of the most relaxed gun laws in the United States: owners are not required to have licenses or register their weapons and there is no limit on the number of firearms an individual can possess.

Once Paddock had purchased the weapons, there was the matter of transporting them to his hotel room. Las Vegas Sheriff Joseph Lombardo said on Monday that Paddock had checked into his room at the Mandalay Bay on September 28, three days before the mass shooting.

Former Las Vegas police officer Randy Sutton—who spent 24 years with the Las Vegas Metropolitan Police Department—told CBS This Morning that Paddock must had made multiple trips to his room, hiding the weapons in luggage.

“He didn’t bring all that up in one trip and he certainly didn’t have bellmen bring it up. Having been there for three days, he had the time to bring that up probably in either luggage or a golf bag or something of that nature,” said Sutton.

“It’s a little surprising that he could’ve hidden it well enough that when the maid service came in that they didn’t see anything but I think this is all part of the investigatory process that they’re going to have to really delve into.”

The shooting has raised questions about levels of security at hotels. Police are yet to confirm how Paddock got the guns to his room and the Mandalay Bay hotel has not commented on its security measures in light of the attack.

But Adam Winkler, a law professor at UCLA and an expert on gun law who has been cited in Supreme Court cases, said that getting guns into a hotel room is relatively easy.

“Most hotels in the United States do not have metal detectors. Anyone can walk into a hotel with firearms without being detected. Firearms in a suitcase would also easily get past any hotel security,” said Winkler.