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.
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.
Precise details of Zazi's exact plans are not yet known because agents continue to investigate and probably de-classify information. But the use of homemade bombs in backpacks, and fertilizer and fuel-packed trucks has been a staple of al Qaeda-linked groups, as well as domestic terrorists in the U.S., for over a decade.
The July 2005 London subway attacks and the foiled 2006 plot to blow up commercial airplanes traveling from the U.K. to the U.S. involved the use of liquid peroxide-based bombs.
In 1995, fertilizer-based bombs were used by Oklahoma City bomber Timothy McVeigh. The 2002 Bali nightclub bombing, the 2003 synagogue bombings in Istanbul and a 2007 planned bombing of shopping malls in London were all fertilizer-based bombs.
Unraveling such basic but destructive plots before they occur presents huge challenges. When arrests are made early in an investigation - which appears to be the situation in the Zazi case - evidence and suspects have a tendency to disappear.
In typical criminal investigations, law enforcement agents continue to investigate a crime as long as they possibly can to obtain evidence and to identify those involved.
I once investigated a man who had savagely killed three people in a highly-publicized robbery. In order to get close to this killer and build the evidence, I sent a police informant to talk to him. I was reasonably certain that, given the media attention the murders had received, the killer would not act again. Since I also had a court-authorized wiretap on his phone, I knew from his conversations that he was not planning any new murders. This gave me the luxury of time to develop the evidence about the murders by using the informant.
I fitted the informant with a transmitter and body recorder and instructed him to purchase drugs from the killer's cohort many times. I hoped to arrest the cohort and get him to testify later against the killer. In order to prove the murder case, I knew we needed more evidence for a judge and jury.