How did the Istanbul airport attackers get past security?

 (Vision)
Terrorists targeted the 11th busiest airport in the world in a country that borders the ISIS-stronghold Syria, and it's not hard to see why Ataturk posed an attractive soft target.
But what's particularly unnerving about the latest incident is the regional aviation hub had a much tighter security system in place than many other airports.
Travelers using the airport are required to undergo double security screenings. One before they enter the international terminal building, and a second time after they go through passport control.

)The deadly assault by three suicide bombers on Istanbul's Ataturk Airport once again shows the difficulty in averting terror attacks.