An Arizona man who sold ammunition to the gunman in the deadliest mass shooting in modern U.S. history has been charged with manufacturing armor-piercing bullets.

A complaint says unfired armor-piercing bullets found inside the Las Vegas hotel room where the attack was launched on Oct. 1 contained the fingerprints of ammunition dealer Douglas Haig of Arizona.

He faces charges of conspiracy and manufacturing armor-piercing ammunition.

The complaint filed Friday in federal court in Nevada says Haig didn't have a license to manufacture armor-piercing ammunition, which is required to manufacturer or sell such ammunition by U.S. law.

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The charges were filed in federal court shortly before Haig and his attorney held a news conference Friday. He said he noticed nothing suspicious when he sold 720 rounds of ammunition to Stephen Paddock in the weeks before the attack that killed 58 people.

An Amazon shipping box marked with Haig's name and Mesa address was found in Paddock's hotel room, filing documents show.

The day after the shooting, Oct. 2, FBI and ATF agents arranged an interview with Haig and a business partner in Phoenix. After initially being unclear about whether he'd met Paddock, Haig told investigators that he'd met the shooter at a Phoenix gun show in early September.

He and his associate clarified days later that they'd met Paddock at a Las Vegas show on Aug. 27. Haig told investigators that Paddock bought several packs of .308 incendiary ammunition that day.

READ: Charging documents filed in federal court

Haig then had contact with Paddock again in Phoenix, but Haig said he spoke to the eventual shooter Sept. 19 and gave him directions to his home to buy ammo.

"Paddock asked to purchase 600 rounds of .308 caliber 'tracer' a,munition and a quantity of .223 ammunition from Haig," filing documents state.

Haig then put the ammunition in the box later found in Paddock's room.

A forensic evaluation showed Haig's fingerprints were found on several of the unfired cartridges found in the hotel room. It also revealed markings consistent with reloading the bullets, which were found to have armor-piercing components inside them.

Haig told investigators that he does reload bullets with armor-piercing components, but documents show investigators found more than 100 instances of sales including various types of armor-piercing ammunition across the country within his sales records.

Additionally, Haig's business partner told investigators that he knew Haig was reloading armor-piercing ammo and selling it.

Haig used his online business Specialized Military Ammunition to sell ammunition. The site, which says it is now closed indefinitely, advertised armor-piercing ammunition when investigators first reviewed it. When investigators visited the site again Oct. 16, that advertisement had been removed.

Haig is a 55-year-old aerospace engineer who sold ammunition as a hobby for about 25 years.

The Associated Press contributed to this report.