WASHINGTON — The bomb explosion had been spectacular, sending shock waves for miles. But Amine El Khalifi, who dreamed of a martyr's death with explosives strapped to his chest, seemed unimpressed.

"This is not enough," Khalifi told two men who he thought were al-Qaida operatives after they demonstrated the bomb's power for him at a West Virginia quarry on a frigid January afternoon.

The men were undercover FBI agents who had spent months getting close to the Moroccan immigrant. That morning, on the way to the quarry, Khalifi had told one of them that he no longer wanted to leave a bomb in a restaurant; he now desired to die for his cause in a suicide attack that would bring down a symbol of American democracy: the U.S. Capitol dome.

A month after the quarry demonstration, Khalifi was arrested as he strode toward the U.S. Capitol wearing what he thought was a suicide coat. He was sentenced in September to 30 years in federal prison after pleading guilty to attempting to use a weapon of mass destruction.

This type of undercover sting operation, in which authorities seek individuals they think would be willing and able to carry out terrorist attacks in the United States, has generated controversy. Civil liberties groups say that the investigations identify people with radical views but who could not attempt an attack without the government's help.

FBI officials, who have arrested scores of suspects in such stings since the Sept. 11, 2001, terrorist attacks, call the investigations a key tool in their efforts to prevent terrorism. They also insist that the investigations are thorough and seriously conducted. Before agents launch one, they spend months determining whether someone is full of bluster or a real threat, bureau officials said.

The vast majority of such inquiries determine that the person is harmless, the FBI says.

"It is your constitutional right to spout anti-American beliefs," said agent Steven Hersem, who helped supervise the Khalifi investigation. "We spend a lot of our time trying to figure out if someone is an actual threat or not."

In Khalifi's case, the conversation before the quarry explosion convinced them.

"That was an epiphany for me that this man is a definite threat and he must be stopped at all costs," said Bryan Paarmann, assistant special agent in charge of counterterrorism at the FBI's Washington Field Office.

Initially, Khalifi was no different from many young men who find their way to the United States. He had been born in Morocco and visited Florida with his father at age 16. He overstayed his visa and eventually moved to Northern Virginia, where he worked odd jobs as a cook, busboy and salesman.

He got into mixing and producing music. For a time, agents said, he was a fixture on the D.C. club scene, where he started to use cocaine, marijuana and ecstasy.

In 2007, Khalifi was arrested and charged with assault after an argument at a club. At some point after the arrest, agents said, he decided to be a more devout Muslim and was drawn to the teachings of radical clerics on the Internet.

An FBI analyst noticed in July 2010 that Khalifi had responded to a Facebook posting by a known terrorist in Afghanistan seeking help for his cause. Six months later, a confidential informant told agents that Khalifi was hanging out with friends — some of whom were on the FBI's radar — in an apartment in Arlington County, Va., when someone pulled out an AK-47 and two revolvers. Khalifi agreed with his friends that the war on terrorism was a war on Muslims and that they should be ready for battle, according to the informant.

The tip and the Facebook posting got the bureau's attention. Agents combed public records and reached out to a network of informants to learn more about Khalifi. The informants told agents that Khalifi wasn't just expressing support for terrorist groups and jihad; he wanted to participate in what he considered a holy war.

Within a month or so, FBI agents decided that Khalifi was dangerous enough to warrant an intensive inquiry. They tapped his phones, monitored his Internet use and tracked his movements with teams of surveillance agents. The work confirmed what their sources had been saying, and FBI officials said it was time to get a better sense of what Khalifi had in mind by sending in undercover agents.

By late summer of last year, agents determined that Khalifi was brokering car sales on the Internet and was looking to buy a Toyota Prius. One of the FBI's undercover agents, a man who called himself "Hussien," posted in an online advertisement that he was selling such a car.

The agent soon heard from Khalifi, and they met in an office building parking lot on a cloudy and warm Thursday in September 2011. They chatted about the car and then hit it off, the FBI said, speaking in Arabic about their pasts as Muslims from Arab countries.

Khalifi also spouted violent rhetoric about Jews, Israel and U.S. policy. Hussien played it loose, agents said, mostly nodding in agreement.

"That was a good first meeting," said the case's lead agent, who spoke on the condition of anonymity for security purposes. "It set up a follow-up meeting. We were establishing a relationship."

The men met again the next day, and Khalifi bought the car. Soon, Hussien and Khalifi were hanging out and chatting on the phone about religion and automobiles. The agents lined up another deal, this time for a beat-up Volkswagen, to further develop the relationship.

By the end of that September, Khalifi was no longer speaking in "nebulous" terms, the FBI agent said. He was doing research on the Internet to justify his desire to engage in jihad overseas and was willing to die for the cause in Afghanistan or for the Palestinians.

Khalifi next told Hussien that he wanted weapons training and had his eye on attacking an office building in Alexandria, Va., that he thought housed military offices. The news concerned FBI officials, who ramped up surveillance of Khalifi to ensure that he didn't try to launch an attack on his own and without the knowledge of undercover agents.

In December, Hussien took Khalifi to a comrade in Baltimore, a person the undercover agent promised could help in his quest for jihad. The other man, who called himself "Yussef," was also an FBI agent. During the encounter, the men handled an AK-47, and Khalifi discussed his desire to shoot someone at the office building or to blow it up.

A week later, Khalifi started talking about attacking a synagogue or killing a U.S. Army general leaving his house. Then he wanted to leave a bomb at Aria Pizzeria, a restaurant at the Ronald Reagan Building in downtown Washington. He and Hussien ate lunch at the spot, where Hussien told Khalifi that he and Yussef were members of al-Qaida.

At nearly every meeting, FBI agents said, Hussien asked Khalifi whether he was sure he wanted to launch an attack and whether there was a more peaceful way to engage in jihad than killing people. In past investigations, other suspects have walked away from similar plots and not been charged, FBI officials said.

At one point, Khalifi became so frustrated by the questions that he told the undercover agent to "stop asking him if he wanted to do this," the case's lead agent said.

On Feb. 17, a month after the quarry explosion, Hussien and Yussef picked up Khalifi in a van, handed him an inoperative Mac-10 submachine gun and helped Khalifi into what he thought was an explosives-laden coat.

During the ride to the Capitol, where he would soon be swarmed by FBI agents, Khalifi remained calm, focused and determined — so much so that Hussien began to weep, suddenly overwhelmed by the would-be terrorist's zeal to kill.