As Hosam Maher Husein Smadi prepared to remotely detonate what he believed was a powerful bomb underneath a Dallas skyscraper, his comrade-in-arms, who was actually an undercover FBI agent, offered him earplugs, authorities say.

Smadi declined. He said he wanted to hear the blast, according to investigators.

Sitting in a car a safe distance from the skyscraper, Smadi, 19, allegedly dialed the cell phone number he thought would trigger the explosion. It was instead a law enforcement number. He was promptly arrested on terrorism charges.

The FBI on Thursday announced the arrests of Smadi and another man, Michael C. Finton, who was charged with trying to blow up a building in Illinois in a nearly identical but unrelated plot.

In both cases, FBI agents posing as Al Qaeda operatives supplied the men with bogus explosives. In both cases, the FBI let the sting operation play out to the very end.

That is a markedly different strategy from the one employed by the FBI in some other big terrorism cases since 9/11, and it could yield the crucial caught-in-the-act evidence that juries like to see.

"Obviously if you nip it too early in the bud, then you may not have credible evidence to use in court," said Scott Silliman, executive director of Duke University law school's Center on Law, Ethics and National Security.

The strategy could also blunt the kind of entrapment claims that have made it difficult to win convictions in terrorist plots that were broken up by the FBI in their very earliest stages.

Letting a sting run its course shows that the defendants "are acting on their own and willing to go forward and you're not guiding them into what they're doing," former FBI terrorism specialist Roger Rehling said. "You get to see their motives and their reactions at what they think is about to happen — how excited they are to be doing it."

Smadi, a Jordanian who lives in Texas, is accused of trying to blow up the 60-story Fountain Place office tower. Finton, a 29-year-old American who went by the name Talib Islam and idolized John Walker Lindh — the American-born Taliban fighter — allegedly used a cell phone to try to detonate what he believed were explosives in a van outside a federal courthouse in Springfield, Ill.

Both are charged with trying to use a weapon of mass destruction and face up to life in prison if convicted. Finton is also charged with attempting to murder federal employees.

Richard Anderson, the public defender appointed to represent Smadi in Dallas, said Friday he had little to say until he figured out more about the case. Finton's attorney, Robert Scherschlight, did not immediately return a call for comment.

The U.S. Attorney's office in Dallas referred calls on the arrests to FBI headquarters in Washington. FBI spokeswoman Denise Balew said the bureau would not comment on any aspect of the case, including the FBI's tactics.

In a similar case in May, four men were arrested after they allegedly tried to detonate fake explosives — also provided by the FBI — outside two synagogues in New York City.

The government has had mixed results prosecuting other terrorist plots that were broken up early.

After a six-month investigation, seven men were arrested in 2006 on charges of plotting with an FBI informant posing as an Al Qaeda operative to blow up Chicago's Sears Tower. Two defendants were acquitted and five were convicted after two mistrials resulting from deadlocked juries. Those juries said they struggled for lack of solid evidence that the men took concrete steps to set the plot in motion.

Last year, five Muslim immigrants were convicted of conspiring to kill U.S. soldiers at Fort Dix, N.J. But they were acquitted of attempted murder after prosecutors acknowledged they were probably months away from acting on the plans.

Donald Young, an Illinois lawyer who represented a Muslim convert accused of plotting to blow up garbage cans at a shopping center during the Christmas season, said that because Finton and Smadi allegedly tried to detonate what they thought were bombs, it will be hard for their attorneys to argue entrapment.

"Entrapment is a very difficult defense in these cases if the government can show that the individual is predisposed to commit the act," he said.

Silliman said the practice of letting a sting play out to the very end does not represent a change in strategy on the government's part.

It's really a case-by-case decision. I wouldn't suggest to you that the Justice Department and the FBI are all of a sudden saying, 'Well, we're going to wait for these plots to develop just so we can prosecute.' They have been very successful in getting convictions in a lot of these terrorism cases in the United States," he said.

More coverage at MyFOXChicago.com.