VIRGINIA BEACH – Toiling away Sunday morning in the muggy confines of his gun shop, Mike Brini said requests for suppressors, a device used by the shooter who killed 12 people in a nearby government office building, are few and far between.

While a suppressor muffles the sound of gunfire, gun rights advocates and law enforcement experts said the use of one likely did not determine the deadliness of Friday's shooting but could be a sign that the suspect carefully planned the attack.

Police say the shooter attached a suppressor, also known as a silencer, to the .45-caliber handgun that he fired on three floors of the building where he worked before he was fatally shot by police.

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

Brini, a gunsmith and owner of Firearm Performance Enhancements located about 3 miles from the site of shooting, said he could only think of two businesses in Virginia Beach with a license to sell silencers.

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While authorities said the suspected shooter legally bought multiple firearms recently, they have not yet said how he got a suppressor. Some gun experts say factors other than the suppressor, such as the shooter’s familiarity with the building and even his military background, played a greater role in the tragedy.

“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.”

A suppressor does not silence the sound of a gun firing but lowers it by 20 to 35 decibels, leaving most guns louder than an average ambulance siren. The National Firearms Act regulates suppressors, and the extensive background check can take eight months or more before the sale can go through.

Plus, said Pete Knott, 53, a Virginia Beach gun enthusiast who stopped by Brini’s shop, getting a silencer is an expensive endeavor. To buy one to use at his hunt club, Knott said he had to hire an attorney to form what is known as a gun trust.

“You can’t buy a Class 3 weapon without a gun trust,” Knott said.

All told, including attorney’s fees, taxes and cost, Knott said he spent about $1,500 to purchase one suppressor. He speculated the shooter at the Virginia Beach Municipal building possibly waited a long time to carry out the shooting, as did other experts.

“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 caliber 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.

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

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It’s not immediately clear how long Friday’s attack lasted, or how much time passed before the first police officers arrived on scene. But some gun control advocates say the suppressor may have caught the victims off guard. One survivor described hearing something that sounded like a nail gun.

“Especially on a handgun, a suppressor will distort the sound in such a way that it would not immediately be recognizable as gunfire to people who sort of know what that sound is,” said David Chipman, a retired agent with the federal Bureau of Alcohol, Tobacco, Firearms and Explosives and now the senior policy adviser with Giffords, a gun-control lobbying group.

Suppressors seem to have grown more popular in the last decade. In 2008, when West Valley City, Utah-based SilencerCo was formed, about 18,000 of the devices were sold each year. The company, which controls an estimated 70% of the market, now sells roughly that many each month.

Lam reported from Los Angeles. Contributing: The Associated Press