The speed, pattern and inconsistent rate of gunfire heard in videos of the Las Vegas shooting indicate the suspect could have used cheap and legal modification devices to accelerate the firing of a semi-automatic weapon to almost 700 rounds a minute.

And the kinds of devices that would make that possible are readily available — relatively cheaply on the internet — even to people who can't qualify to buy an automatic weapon.

The shooter purchased at least some of his weapons from Guns & Guitars in Mesquite, Nev., said Chris Sullivan, the shop's owner, who issued a statement mourning the victims of the attack.

Guns & Guitars does not sell fully-automatic rifles and is not a class III license holder, he said.

"Mr. Paddock was a customer and purchased firearms from our store; however, all necessary background checks and procedures were followed, as required by local, state, and federal law. He never gave any indication or reason to believe he was unstable or unfit at any time," Sullivan wrote in a statement to USA TODAY.

Video taken near the Mandalay Bay Casino by a bystander provides audio of the continuous fire. While fast, the audio doesn’t mimic a consistent, fully automatic rate, said Mel Bernstein, owner of Dragonman’s shooting range in Colorado Springs, Colo., who analyzed the sounds from the video for USA TODAY.

USA TODAY reviewed the video and counted more than 10 shots fired per second.

Semi-automatic weapons fire one round per trigger pull, while fully-automatic rifles fire continuously when the trigger is depressed. It’s also possible the shooter had fully-automatic weapons, which are only legal with a special permit that requires fingerprinting and federal approval.

Two legal, off-the-shelf devices could have been used to recreate a fully-automatic rate of fire, increasing the speed by hundreds of rounds per minute.

One, known as a “trigger crank” or “gat crank” bolts onto the trigger guard of a semi-automatic rifle. The shooter then rotates the crank, which usually depresses the trigger three times per rotation, Bernstein said.

The trigger cranks sell online relatively cheaply.

“They really shouldn’t be legal — anybody with an AR-15 can bolt one on and crank out rounds as fast as a fully-automatic without a class III machine gun license,” Bernstein said, adding that he doesn’t sell the devices at his shop in Colorado.

Another modification to a semi-automatic weapon that could have been used in the shooting is what’s called a “bump stock.”

The device modifies the stock of the gun so the recoil helps fire rounds in rapid succession. The bump stock is also legal in the U.S.

“You can take an M-16 and it fires about the same as a bump fire—it goes with your body back and forth,” Bernstein said.

The shooter could have used semi-automatic weapons with legal modifications, or a fully-automatic weapon with, or without a permit, said Mike McLively, a staff attorney at the Law Center to Prevent Gun Violence.

“The only issue with the modifications is you lose accuracy, but of course if you’re aiming at tens of thousands of people in a crowd, accuracy doesn’t matter as much,” McLively said. “If it turns out a modification was used, I think it’ll start a conversation about whether they should be legal. People think it’s fun to shoot a lot of rounds—but you have to weigh that against the 500 people injured and nearly 60 people killed.”

Clark County Sheriff Joe Lombardo identified the shooter as Stephen Paddock, 64, who the sheriff said was found dead by officers after they stormed into his hotel room on the 32nd floor of the Mandalay Bay Resort and Casino.

Records about the named suspect's firearms licensing have not yet been publicly revealed.

Kristen Rand, legislative director for the Violence Policy Center, says there are a variety of tools on the market for speeding up the number of rounds fired by semiautomatic weapons.

Rand said the Bureau of Alcohol, Tobacco, Firearms and Explosives reviews devices individually to determine whether they function as a machine gun, or whether the action pulls the trigger independently for each round. The ATF has said that many of these function, technically, as a semi-automatic weapon -- making them legal.