Survivors were initially puzzled by what was happening, describing hearing something that sounded like a nail gun.

The attacker who killed 12 people in a government office building in Virginia Beach city of the United States used a firearm equipped with a “silencer” that muffles the sound of gunfire.

It is the nightmare scenario that gun-control advocates have warned about amid efforts in recent years to ease restrictions on the devices, which they say can help shooters escape detection and inflict more carnage.

But gun-rights advocates and most law enforcement experts say DeWayne Craddock’s use of a suppressor likely had no bearing on his ability to kill so many people in so little time on Friday.

Virginia is among 42 states that allow residents to buy and possess suppressors, though some cities and towns – including Virginia Beach – prohibit them.

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Known colloquially as a silencer, a suppressor was attached to the .45-caliber handgun that police say the shooter used to kill a dozen people on three floors of the building where he worked before police closed in and, after a protracted gun battle, fatally shot him.

‘Sounded like a nail gun’

That could at least partially explain why survivors of the attack said they were caught off-guard and initially puzzled by what was happening. One described hearing something that sounded like a nail gun.

“This is the concern we were talking about when Republicans were trying to deregulate silencers as ‘ear protection’,” said David Chipman, a retired agent with the federal Bureau of Alcohol, Tobacco, Firearms and Explosives and now senior policy adviser with Giffords, a gun-control lobbying group.

“Especially on a handgun, a suppressor will distort the sound in such a way that it would not immediately be recognisable as gunfire to people who sort of know what that sound is.”

Others say the shooter’s use of a silencer was less of a factor in enabling him to carry out the rampage than was his familiarity with the building and even possibly his military background, both of which may have given him a tactical advantage.

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“A suppressor does not alter the lethality of the weapon at all. All it does is just limit the noise it makes,” said Gregory Shaffer, a retired FBI agent who was a member of the bureau’s elite Hostage Response Team.

“It doesn’t increase the rate of fire. It doesn’t do anything other than make it more comfortable to shoot because it’s not so loud.”

It’s not immediately clear how long Friday’s attack lasted, or how much time passed before the first police officers arrived. The police department is in the same complex as the building where the shooting took place.

It also wasn’t yet known how Craddock got the suppressor he used on his handgun, though authorities have said he legally purchased multiple firearms recently.

Gaining in popularity

Authorities have three days to conduct a background check when someone is buying a firearm. But suppressors are regulated by the National Firearms Act, which also governs the sale of machine guns, and the extensive background check can take upward of eight months or more before the sale can go through.

Despite the barriers, suppressors have gained in popularity. In 2008, when West Valley City, Utah-based SilencerCo was formed, about 18,000 of the devices were being sold each year.

Silencers on display at the Shooting, Hunting and Outdoor Trade Show in Las Vegas in 2016 [File: John Locher/AP]

The company, which controls an estimated 70 percent of the market, sells roughly that many each month.

Nicknamed “cans”, the devices were invented in the early 1900s by MIT-educated Hiram Percy Maxim, who also invented a muffler for petrol engines. They were brought under NFA regulations after Depression-era game wardens expressed concern that hunters would use them to poach.

A suppressor does not eliminate the sound a gun makes but generally diminishes it by 20 to 35 decibels, leaving most guns still louder than your average ambulance siren.

“Clearly, this was an individual who did understand and have experience with firearms and had given potentially some forethought into the advantage that using a suppressor would offer him, particularly the suppressor coupled with the calibre of weapon he was using,” said Thor Eells, executive director of the National Tactical Officers Association and a retired law enforcement officer with the Colorado Springs Police Department, where he oversaw a SWAT division.

Some have questioned how secure the building was where police say the attacker and all but one of his victims worked.

A government facility, the building is open to the public, but security passes are required to enter inner offices, conference rooms and other work areas, officials said.

As a current employee, the attacker would have had such a pass and would have known the floor plan, areas that were “easy to control”, where the best places to hide were and how to move quickly from one area to another, Eells said.

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While responding, police might have had some familiarity with the building, it’s very possible the attacker knew it a lot better after working there for years.

His protracted gunfight with law enforcement officers would indicate that he “was in a place that was difficult for officers to access or engage”, Eells said.

“Whether that was happenstance or intentional, it’s too early to tell.”