Several experts and retired law enforcement we spoke with say the sound of the gunfire in the Las Vegas shooting suggests at least one fully automatic or converted weapon was used.
Fully automatic rifles can be legally bought, but they are heavily regulated.
The sound, the cadence of rapid gunfire in Vegas, is a sound that has retired FBI agent Brian Kensel convinced the shooter was armed with at least one or more fully automatic rifles or conversion weapons.
Kensel spent 24 years with the FBI and most of his career on the SWAT team. He says a fully automatic weapon can be bought legally but there are extra steps to take.
"It's not terribly difficult. It is terribly expensive, you can spend about $20 thousand," he tells us.
The weapons are expensive because supply is limited.
Any fully automatic firearm manufactured or imported after 1986 can only be purchased by a properly licensed gun dealer, law enforcement, or the military.
Owners of fully auto guns have to pay a special $200 tax, as well as go through a background check, fingerprints, and get written approval from a local police chief or sheriff as well as fill out ATF forms.
Kensel explains, "some states or even cities or counties put additional restrictions, some of them ban outright, even if it's legally owned under Federal law.
There are conversion kits or devices you can buy online or the black market to convert a semi automatic weapon into a near or fully automatic gun.
"It's against the law you just cant modify, trick out, or change," says Kevin Richardson, a retired ATF agent.
Richardson spent 27 years with the ATF. He was on scene at the Pulse nightclub shooting in Orlando. He says while ATF doesn't have a record tracking system on firearms, there are ways to track forms associated with the transfer and the special $200 tax paid. Something he believes ATF agents are already doing in Vegas.
"They probably have that info already. The information is there."
Experts say even on the black market, fully automatic rifles are hard to get.