Authorities have not officially released details on the weapons a gunman used to kill 58 people and wound about 500 more late on Sunday in Las Vegas. But analysis of video posted on social media shows that the gunman, identified by the police as Stephen Paddock, 64, had rifles with rapid-fire capabilities.

An arsenal of firearms was recovered from Mr. Paddock’s hotel room, said Sheriff Joseph Lombardo of the Las Vegas Metropolitan Police Department. At least one rifle in Mr. Paddock’s suite had a “bump stock,” a device used to retrofit a semiautomatic firearm to make it function like a fully automatic weapon, according to a law enforcement official who requested anonymity to divulge details of the investigation.

This video shows 15 seconds of the attack, with constant gunshots ringing out.

Isolated, the pattern of gunshots looks like this.

Las Vegas About 90 shots in 10 seconds

Compare that with audio extracted from a video of the June 2016 Orlando nightclub shooting, in which 49 people were killed and 53 were wounded. The gunman, Omar Mateen, used at least two guns, including a semiautomatic AR-15-style assault rifle.

Orlando nightclub 24 shots in 9 seconds

In contrast, a fully automatic weapon, like this pre-1986 Colt AR-15A2, sounds different. There are no variations in the firing rate like there was in the Las Vegas shooting.

Fully automatic weapon 98 shots in 7 seconds

How did he fire so quickly?

The Las Vegas gunman modified at least one of his semiautomatic rifles to fire faster using a bump stock.

Slide Fire, a main manufacturer of bump-fire stocks, shows how the product works in the promotional video below.

Promotional video by Slide Fire

The device replaces a rifle’s standard stock, which is the part held against the shoulder. It frees the weapon to slide back and forth rapidly because of recoil, with the stock bouncing, or “bumping,” between the shooter’s shoulder and trigger finger.

The shooter holds his or her trigger finger in place, while maintaining forward pressure on the barrel and backward pressure on the pistol grip while firing. This modification can be done legally and allows the user to fire at rates similar to those of automatic weapons.