
Editor's note: Former FBI Agent Brad Garrett is an ABC News consultant and regular contributor to the Blotter.
At 8:30 any given weekday morning in New York City, an estimated five million people are riding the city's subway system. They are a prime and vulnerable target.
For the FBI and the New York police department, it is a nightmare scenario that agents and detectives have played out in their planning.
Terrorists bent on destruction could enter nine of the city's 425 subway stations wearing backpacks filled with homemade bombs and face little risk of detection.
As was made clear in court documents in the most recent alleged terror case, bombs can easily be constructed with over-the-counter products used to style hair or remove nail polish.
With enough followers, a terrorist mastermind has no need to limit the attack to just the subways.
The city's major international financial institutions present another inviting target for terrorists seeking a one-two punch. Ten rental trucks — or even coffee carts - packed with easily purchased fertilizer, diesel fuel and cotton would create even more destruction and death.
My former law enforcement colleagues say this is the chilling scenario of what Najibullah Zazi and his associates might have done.