It's the type of scenario security experts dread: attackers blending in with the crowd at public places to carry out acts of mass murder.
That scenario appears to have played out again Tuesday, this time at an Istanbul airport.
The ability to defend "soft targets" — places where it's relatively easy for a small group of terrorists to cause widespread and deadly devastation — remains elusive.
In the U.S., security officials and experts have long worried that these types of places are extremely vulnerable.
"If you have a determined terrorist who is willing to give up their life … it's almost impossible to prevent that across the board," John Pistole, a former administrator with the Transportation Security Administration, told NBC News in March.