(CNN) We know that Las Vegas shooter Stephen Paddock assembled a small arsenal in room 32135 of the Mandalay Bay Resort and Casino, where investigators found 23 weapons, including multiple rifles and a handgun.

What kinds of guns were they? Authorities haven't told us yet.

But the sounds of gunfire from videos of the attacks offer a clue.

What automatic fire sounds like

It's obvious that the gunfire is automatic fire, retired ATF special agent Sam Rabadi told CNN's "New Day" after anchor Alisyn Camerota played a short video of the shooting. She said experts who analyzed it said you can hear 90 gunshots in just 10 seconds.

Richard Vasquez, a former chief of firearms technology for the ATF, said "without a doubt it was automatic fire," but it's hard to determine any more specifics about the weapons used because the videos are shot from so far away and there are lots of gunshot echoes.

The difference between semi-automatic weapons and automatic weapons is what happens when you pull the trigger . On semi-automatic weapons the trigger has to be pulled every time to fire a round. On automatic weapons, the trigger has to be pulled and held only once to fire off several rounds.

Sales of automatic weapons are banned in the US. Sales of semiautomatic, or assault weapons, are legal in most states, including Nevada

But just because the gunfire sounded automatic doesn't necessarily mean that Paddock used automatic weapons.

It's possible Paddock took the semi-automatic weapons he was known to have bought legally and used special tools to convert them to automatic weapons, Rabadi said.

How to make guns automatic

"There are a number of ways to do it," he said. "There are some accessories that are available on the market, such as a bump fire stock or a slide fire. That is an accessory that can be used to modify a weapon like an AR-15 to allow it to fire in rapid succession or automatic fire."

These tools are legal and can be purchased online or from a manufacturer, Rabadi said, who explained that the accessories work by using "the gun's own inertia to slide back and forth and allow the firing of the weapon in rapid succession."

These accessories are legal under the National Firearms Act, but Rabadi noted they didn't exist when the law was enacted in 1934.

"As technology has come through in the last number of years I believe the original authors of the act probably did not envision this kind of accessory being used for modifications of this sort," he said.

Rabadi also said such tools are often used by sportsmen who want to modify their weapons for activities like target practice.

Twelve bump fire stocks were found on firearms recovered from Paddock's hotel room, said Jill Snyder, the special agent in charge of the Bureau of Alcohol, Tobacco, Firearms and Explosives' San Francisco field office.

Snyder said authorities are still determining which firearms were used in the shooting.