The man seized for allegedly trying to blow up the Federal Reserve Bank in New York wanted to "destroy America" and said al Qaeda inspired him. When the FBI got wind of his plans, they brought him down in a sting.

Federal authorities nabbed 21-year-old Quazi Mohammad Rezwanual Ahsan Nafis on Wednesday and said he tried to detonate what he thought was a 1,000-pound bomb.

Using his cell phone as a trigger, they say, Nafis sat in a hotel near the bank, where "the world's largest accumulation of gold" -- belonging to 36 foreign governments, central banks and international organizations -- is secured in a vault 80 feet beneath the ground floor.

But the device was actually inert and part of an elaborate undercover investigation by anti-terrorism teams and New York Police Department detectives.

It's unclear whether Nafis -- who came to the United States on a student visa from Bangladesh -- maintained al Qaeda ties, but authorities say he apparently claimed that the plot was his own and that it was his sole motivation for the U.S. trip.

News of the plot prompted lawmakers, like U.S. Sen. Charles Schumer, a New York Democrat, to call for tighter student visa regulations and renewed questions about the city's vulnerability to attacks.

It's the kind of case, like other terror plots, that gives police and attorneys headaches as they try to tell barroom bravado from a sincere resolve to strike.

A paper tiger or a real threat?

Individual plotters with an al Qaeda mindset, as Nafis is alleged to have, must look for help, because most of the time, they don't have the resources or the gumption to carry out a plot, according to New America Foundation counterterrorism analyst Phil Mudd.

And that, he says, makes them "vulnerable to penetration" by law enforcement.

In cases like these, agents want to let the case evolve instead of arresting the suspect before the plot plays out, to see whether the suspect is just a paper tiger or a real threat, Mudd explained.

"You don't want to just understand him; you want to understand whether there's a broader conspiracy here," said Mudd, a former FBI and CIA deputy director. "You want to understand: Where's the money? Who recruited him? Who did he recruit? ... If you move too quickly, you risk leaving some pieces of this on the cutting room floor, and that's a real problem for law enforcement."

But that creates another issue: Some terror suspects lacked the ability to carry out their aims until they were helped along by police.

For example, CNN National Security Analyst Peter Bergen cited a 2009 case he described as "problematic": a plot to detonate bombs at two Bronx synagogues and attack a military base in New York.

The men sentenced in that plot were penniless and ensnared by an informant who promised them material goods, Bergen said. A defense attorney for one of the men criticized the government and claimed entrapment.

But police and federal officials in this and other terror plot cases have been successful in picking their targets and ultimately making their arguments in court, according to numbers from Jennifer Rowland, a program associate at the New America Foundation think tank.

Since the September 11 attacks, most of the jihadist terror suspects in cases with an informant or an undercover agent have been convicted.

Of 94 people indicted, Rowland said, 77 went to trial and were convicted, four pleaded guilty, 11 are awaiting trial, one is dead, and one is at large.

There have been more than 40 such cases or plots within those 94 indictments, she said.

Charges were dropped in June against a South Florida man accused of raising money for the Pakistani Taliban, Rowland said.

What will happen to Nafis? Bergen said he was clearly enabled. But it appears that "he did come to the United States to do jihad."

How the New York bank plot unfolded

Nafis apparently contacted an FBI source and proposed multiple targets, including a high-ranking U.S. official as well as the New York Stock Exchange, authorities said. At one point, the suspect contemplated President Barack Obama as a target, but that idea never progressed, a U.S. official with knowledge of the investigation said.

Nafis, who was listed as attending classes in Manhattan, indicated that he wanted to "destroy America" by going after the nation's financial institutions and ultimately settled on the landmark bank.

An undercover agent, authorities say, also provided 20 bags of 50 pounds each of purported explosives to Nafis, who then stored the material in a warehouse in preparation for the strike.