LA FERIA – Las Vegas law enforcement officials confirm at least 12 of the weapons in the shooter's hotel room was modified with a bump stock.

"I can't give you an answer on whether any of them are automatic or not, but we are aware of a device called a bump stock," Las Vegas Sheriff Joseph Lombardo told reporters on Tuesday.

A bump stock is a gun modification accessory built to increase a gun's rate of fire.

Rio Grande Valley gun store owner Brian Guerra tells CHANNEL 5 NEWS gun owners across the county can legally obtain them.

"Keep in mind that all of these add-ons to these weapons have all been approved by the federal government," said Guerra. "None of these are illegal. None of these are restricted."

Guerra demonstrated how weapons modified with bump stocks compare to fully automatic rifles. He's licensed by the federal government to manufacture and sell fully automatic weapons to law enforcement.

"So I'm going to take you through what regular semi-auto is like, then we're going to go to semi auto with the bump stock and then I'll show you full auto," said Guerra.

The standard semi-automatic AR-15 Guerra fired dispersed 30 rounds in about six seconds. That's an approximate discharge rate of about five rounds per second.

Guerra then demonstrated the AR-15 modified with a bump stock. It had a significantly faster firing rate of about 7.5 rounds per second. The bump stock weapon simulated automatic fire, though didn't completely replicate it.

This weapon didn't carry the consistent firing pattern characteristic of fully automatic weapons.

The fully automatic AR-15 cleared a 30 round magazine in less than two seconds with a single pull of the trigger. This is an approximate firing rate of at least 15 rounds per second.

Guerra tells CHANNEL 5 NEWS videos emerging from Las Vegas lead him to believe at least some portion of the attack was carried out with modified semi-automatic weapons.

"Full autos are very well tuned machines and they have a very consistent shot pattern sound," said Guerra.

Obtaining these weapons is difficult for civilians, although there are legal means.

Investigators are still working to determine if any of the weapons used in the attack were fully automatic.