Frank Geiman of Baltimore fires an AR-15 semiautomatic rifle at the annual Machine Gun Shoot at the Knob Creek Gun Range in West Point, Ky., in 2002. (John Lok/AFP/Getty Images)

The fusillade emanating from the Mandalay Bay hotel in Las Vegas on Sunday night sounded to many as if it came from one or more automatic rifles, which fire continuously as long as the trigger is held down. Such guns are legal, as long as they were made before May 1986 and are registered with the federal government.

If an automatic weapon, also called a machine gun, was made or imported after 1986, it may be legally owned only by licensed dealers, police and the military.

Congress began regulating such weapons under the National Firearms Act in 1934, in response to criminals having greater firepower than the police. Owners of automatic weapons were required to pay a $200 tax, a large amount at the time, as well as provide fingerprints and a photograph, undergo a background check and obtain approval from the chief law enforcement officer in the area. Except for the local police approval, those requirements remain in place, and the $200 charge has not changed.

Semiautomatic guns, which fire only once for each trigger pull, may not be legally modified to automatic. And anyone who wants to buy an automatic weapon must undergo the Bureau of Alcohol, Tobacco, Firearms and Explosives background check and registration process. But there are plenty of automatic weapons available for sale on the Internet. Guns made before 1986 may be owned by anyone who passes a background check and registers the gun. A letter from ATF to the National Firearms Act Trade and Collectors Association last year indicated that there were 490,664 automatic weapons in ATF’s National Firearms Registration and Transfer Record system.

Police found 12 weapons with bump-fire stocks in the shooter's hotel room. These devices can be used to make semi-automatic weapons perform like machine guns. (Patrick Martin/The Washington Post)

No official announcement has been made yet about what type of gun or guns was used by Stephen Paddock, the suspected Las Vegas shooter. “It certainly sounds to me like it could be a machine gun,” said Raymond Rowley, a retired ATF agent and law enforcement consult, “but I’d be cautious. Echoes or ricochets could create an anomaly.” A binary trigger, in which a semiautomatic trigger fires once on the pull and again on the release, also enables rapid fire if pulled continuously.

“The use of a regulated machine gun in a crime is very rare,” Rowley said. Paddock could have legally registered and possessed such a gun, he said, or it could be registered but stolen, or smuggled into the country. Or it could have been illegally modified from semiautomatic to automatic. Rowley said ATF undoubtedly was tracing the origin of the gun or guns and would have results soon.

In 1986, Congress passed the Firearm Owners’ Protection Act, which was signed by President Ronald Reagan. It restricted ownership of automatic weapons to those made and registered before 1986 and said they may be manufactured only for use by a government agency.