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.
Unraveling Terror Plots
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.
If at any point during the investigation I thought the killer was going to harm the informant or anyone else, I would have arrested him immediately and taken my lumps in court for having brought a less-developed case. Fortunately, I was able to use the next six months to build an airtight case against the killer.
Concern for Public Safety
It seems reasonable to assume that the agents in the Zazi case acted out of concern for public safety, possibly because they had a foreign intelligence wiretap on Zazi's phone. If they had information that Zazi was going to harm citizens - whether from an informant or a wiretap - they had to make an immediate arrest.
Unlike my murder case, they may not have had the luxury of time to finish their investigation before they took Zazi into custody. If the evidence presented against Zazi in court turns out to be less strong than it might have been with the luxury of time, but the agents averted another 9/11 when they arrested him, any criticism of them at a later time will ring hollow indeed.
The law enforcement and the intelligence communities have been dramatically overhauled since 9/11. Whether these improvements resulted in uncovering Zazi's alleged terror plot is unknown. The 9/11 Commission report noted that the FBI had a limited ability to collect and analyze intelligence, and share evidence with intelligence gathering agencies. The FBI might have disrupted the 9/11 attacks had they connected the movements of Mohammed Atta and the other hijackers through intelligence sharing.
The FBI of today does collect and share intelligence. Perhaps the new FBI could have discovered that three of the four Hamburg, Germany cell members had arrived in the U.S. during the summer of 2000 and begun pilot training.
Apparently the Zazi plot was uncovered because of these improvements. If so, this will be a welcome change.