Two days after Stephen Paddock killed dozens and injured hundreds more in a hail of bullets from the window of a Las Vegas hotel room, far more is known about how he committed the deadliest mass shooting in modern U.S. history than why he did it.

The image of Mr. Paddock painted by acquaintances and family in U.S. media reports has been puzzlingly ordinary – a sharp contrast from the killer who showered gunfire on a country-music festival on Sunday night before shooting himself dead as police converged on his 32nd-floor suite.

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But if the wealthy gambler and happily retired accountant is hard to square with the notorious mass murderer, his personal armoury of guns, ammunition and explosives point to weeks of cold-blooded planning on the part of the resident of Mesquite, Nev.

Investigators found 23 guns in his room at the Mandalay Bay Resort and Casino, media reports said Tuesday, including a range of military-style rifles such as the DDM4, the AK-47 and the AR-15. Sheriff Joseph Lombardo of Clark County, Nev., said Mr. Paddock had scopes for some of his weapons.

Authorities said on Monday that Mr. Paddock transported his arsenal to his room using more than 10 suitcases before the attack. After breaking the windows of his suite in two places with a hammer, he fired on the crowd of more than 20,000 for about nine minutes.

At a press conference on Tuesday, Mr. Lombardo said Mr. Paddock had cameras inside and outside his room during the shooting.

Police found a further 19 firearms at Mr. Paddock's home in Mesquite, along with thousands of rounds of ammunition, and pounds of the exploding target material Tannerite. An undisclosed quantity of ammonium nitrate, a common component in homemade bombs, was found in Mr. Paddock's car.

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Another cache of five handguns, two shotguns and a "plethora" of ammunition was discovered at a home of Mr. Paddock in Reno, Nev., Mr. Lombardo said. That brings Mr. Paddock's known stock of firearms to 49.

The size of his arsenal suggests the shooting was "preplanned extensively," said Mr. Lombardo, who added that the speed of the police response saved "hundreds" of lives.

Reports from three gun shops in Nevada and Utah say that Mr. Paddock bought many of his guns legally, passing federal background checks. Nevada does not require the registration of firearms or a permit to buy them, according to the National Rifle Association.

Even as details of his weapons stockpile add up, facts pointing to Mr. Paddock's motive for the massacre remain elusive. It emerged on Tuesday that he had worked for the U.S. Postal Service and the Internal Revenue Service decades ago.

A 2013 study by the Congressional Research Service found certain demographic and psychological trends in the previous 30 years worth of data on American mass shooters. Mr. Paddock does not appear to fit the bill.

The study found that the shooters tended to be young men with an acute interest in weapons; Mr. Paddock was 64 and until recently was not known by his family to be a gun enthusiast.

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Mass shooters also tend to feel aggrieved by life and dwell on past slights, believing themselves to be on the outside looking in, the study found; by all available accounts, Mr. Paddock was content with the simple life he led, satisfying his gambling habit with a small fortune accumulated through real estate investment and dispersed during frequent trips to Las Vegas, just over an hour's drive from the suburban home he shared with a long-time girlfriend.

Mr. Lombardo said on Tuesday that the girlfriend, Marilou Danley, 62, was currently in the Philippines and was considered a "person of interest" in the investigation, although police said on Monday that she is not believed to have been involved in the attack.

NBC News reported on Tuesday that Mr. Paddock wired $100,000 to a bank account in the Philippines in the days leading up to the shooting.

With reports from wire services