Douglas Haig, 55, of Mesa, Arizona, is the first person arrested and charged in connection with the 1 October massacre, which ranks as the deadliest mass shooting in modern US history.

The gunman, Stephen Paddock, strafed a crowd of concert-goers from his highrise suite at the Mandalay Bay hotel, then killed himself before police stormed his room. No clear motive for the massacre has been determined.

According to the criminal complaint against Haig, filed in US district court in Phoenix, he met with Paddock on more than one occasion, including once at Haig’s home the month before the shooting to sell ammunition to Paddock, the US attorney’s office in Las Vegas said.

It said Haig previously ran an internet business selling armour-piercing bullets – some consisting of high-explosive and incendiary rounds – throughout the United States but lacked a licence to manufacture the ammunition.

Haig is charged with a single count of conspiracy to manufacture and sell armour-piercing ammunition, which carries a maximum penalty of five years in prison and a $250,000 fine, according to the statement.

Prosecutors said Haig’s fingerprints were found on some of the unfired high-calibre rounds at the crime scene and that armour-piercing casings recovered from Paddock’s hotel room bore tool marks matching the “reloading” equipment they said Haig used to assemble ammunition cartridges.

Haig made an initial court appearance before a federal magistrate in Phoenix and was freed under conditional release pending a 15 February status conference set for the case, prosecutors said.

In addition to the 58 people killed by Paddock in the Las Vegas massacre, nearly 500 people were injured, some by gunfire, others trampled or otherwise hurt while running for cover.

Police said Paddock had equipped 12 of the weapons found in his room with bump-stock devices that enable semi-automatic rifles to be fired like fully automatic machine guns.

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