An active shooter killed 12 people at a municipal building in Virginia Beach last Friday. The suspected shooter, a civil engineer who worked in the building, was armed with a .45 caliber handgun with a "silencer" or suppressor attached.
A suppressor does not eliminate the sound a gun makes, but it diminishes it by 20–35 decibels, leaving the gun still louder than a typical ambulance siren, the Associated Press reports. However, the suppressor would distort the sound of a handgun enough that it would not be immediately recognizable as gunfire.
The muffled gunshots initially confused employees in the municipal building, one of whom said it sounded like a nail gun, rationalizing it as part of a workplace renovation project instead of an emergency.
While the debate continues over how much of an impact the suppressor had on the shooting, the suspect's insider knowledge and access to the building enabled him to carry out the attack and move around quickly. The building was open to the public, and all the shooter needed to access the office spaces was his security pass—common issue to any other civil servant on the campus.
The motive for the shooting is currently unclear; the suspect turned in his two-weeks' notice earlier on Friday, but left no note or communication about his frame of mind or intention.
ASIS International has compiled a page of resources about active assailant incidents and preparedness, available here.