Well, as last weekend's failed Times Square car bombing clearly demonstrated, that concern is no longer hypothetical - it is becoming a reality. Just last year, 41 Americans were accused of aiding terrorist groups or plotting terrorist attacks against the U.S. or its allies.
Department of Homeland Security Secretary Janet Napolitano said earlier this year homegrown terrorism is here, and now part of the threat picture we have to confront.
The explosive-laden vehicle that was discovered in Times Square was something that New Yorkers and their police department had been expecting, they had been waiting and on the lookout for years.
New York's Police Commissioner Raymond Kelly told "60 Minutes" correspondent Steve Kroft car bombs are one of the things that worry him the most. "They're mobile. They're relatively easy to put together. You don't need much space. You can do it by yourself."
"We were lucky," Kroft remarked.
"Yes," Kelly acknowledged. "A lot of good investigative work here, but we were lucky. No question about that."
Kelly says the poorly constructed device might have killed hundreds of people if it had gone off. Instead, it took the NYPD and the Joint Terrorism Task Force just 53 hours to identify and arrest the man who built it. According to the complaint, 30-year-old Faisal Shahzad, a freshly minted U.S. citizen who had lived here for a decade, slipped off to his native Pakistan where he received explosives training, and returned to the U.S. on his on his American passport to carry out the attack.
"That is sort of a gold standard," Kelly told Kroft. "That's what al Qaeda and their surrogates look for. Someone who can travel. Someone who has clean paper. Someone who can go to Pakistan, come back and get into the United States because they're a citizen or have resident cards that get 'em back into this country. That gives them tremendous flexibility."
Asked if Shahzad was on anybody's radar, Kelly said, "No, it doesn't appear that way."
"Anything remarkable about him that would lead you to believe he might do something like this?" Kroft asked.
"No, that's a good word, unremarkable. These are unremarkable young men who make a decision to kill innocent people in their own country," Kelly said.
Asked what Shahzad has said about his motives, Kelly said, "He made a general statement that he did it because his religion was under attack."
Web Extra: Who Is Faisal Shahzad?
It is all very similar to the story of Najibullah Zazi. He was an immigrant from Afghanistan who spent much of his life in the United States, leading what seemed like a perfectly normal existence until he traveled to Pakistan in 2008 for explosives training and was later arrested in the U.S. for plotting to set off bombs in the New York City subway system.
"This is somebody that has lived here for ten years, loved playing basketball with his friends. He's a big baseball fan," attorney Art Folsom said.
Folsom was Zazi's lawyer when he began his plea negotiations with the FBI, and he says his client told them that he wanted to retaliate against the U.S. military for the innocent civilians that had been killed in his native country.
"Did you or anybody ask him if you had these feelings about the American participation in the death of innocent civilians in Afghanistan, why didn't you just speak out against it?" Kroft asked.
"He actually did bring that up. He said that if he just sat there and wrote letters to the editor, who was going to listen to him?" Folsom said. "But if he walked into a Walmart with an explosive strapped to him and blew it up, then people were going to pay attention to what it was he had to say."
But Zazi didn't reach that conclusion all by himself. He had originally gone to Pakistan to try and hook up with the Taliban to fight American troops in Afghanistan, but once there he was recruited by al Qaeda and they had bigger plans for him.
Web Extra: Al Qaeda's New Recruits
"They realized they could do more with him in the United States. And I think that they're probably very good at taking people who already have some anger or some passion about something and taking that and twisting it to go in the direction they want it to go," Folsom said.