LAS VEGAS (FOX NEWS) -- Las Vegas gunman Stephen Paddock bought more than 30 weapons and had more than a dozen stashed in his hotel room, as he carried out the deadliest mass shooting in U.S. history, Fox News learned Monday.

The deadly cache included converted, fully automatic AR-15 style assault rifles with high capacity magazines, according to a law enforcement source. The weaponry suggests Paddock passed numerous FBI background checks.

The source said police found between 11 and 15 guns in his hotel room.

Weapons found were both .308 and .223 caliber, Fox News has learned.

Country music star Jason Aldean was on stage Sunday night at the Route 91 Harvest Festival when Paddock opened fire across the street from inside the Mandalay Bay Hotel and Casino, killing at least 58 people and injuring more than 500.

Paddock killed himself as police stormed his hotel room.

One witness told a local television station that he heard "hundreds of shots." The gunfire was rapid and reportedly confused with firecrackers.

"It sounded like a machine gun," one vendor told Fox News. "It sounded like more than one machine gun."

The .223 is known as a typical AR-15 round. It would stop after hitting one adult and is common in small game hunting.

The .308 is a heavy, long range bullet, meant for big game like elk and black bears. It is capable of penetrating 2 people. It is commonly used in an AR-10 rifle.

This may explain the number of injuries. Fully automatic weapons are illegal, but machinists can convert them. YouTube videos also explain how it can be done.

There is one unlikely exception. A machine gun purchased prior to 1986 is eligible for a license from the Bureau of Alcohol, Tobacco, Firearms and Explosives, which issued a tax stamp. But those stamps are almost impossible to acquire.

According to Fox News, some of the weapons were bought in California.

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