>>>BOSTON - Before the September 11, 2001 attacks, Americans mostly thought of terrorism as something that happened overseas. But the response to the Boston marathon attack - and to recent mass shootings - suggests the feeling of safety has been punctured.

Boston from some vantage points can appear idyllic, and that's usually the feeling when the marathon is run. Tracy Shea was there and was loving it.  

“The day was so joyful," she recalled. "It was such a joyful day, and I was so in the moment of the elation of seeing people finish."

Shea got close enough to the finish line to take a picture, when the bombs went off.

“All I saw was just this smoke, it reminded me of 9/11, just seeing that white smoke and the stuff in the smoke coming at you," she said, thinking back to the moment of the explosion. "Then you realized instantly that it was a bomb. And I looked at my daughter who had terror in her eyes and we just started running."

But reality had not sunk in.

“It was really, like, I cannot believe this is happening. I think it was just utter shock," Shea admitted. " 'Are we really running away from bombs on Boylston street?' It was just sheer disbelief."

It's a disbelief that's rooted in a time when Americans felt their country was immune to political violence - especially to the kind of carnage that happened here this week.

“Prior to 9/11, most Americans thought we lived in a security bubble, almost like we were on some other planet and we watched terrible things happen on TV, so they happen in Pakistan, or in Iraq or in Afghanistan or in Africa,” explained Graham Allison, director of the Belfer Center for Science and International Affairs at Harvard University.

That's changing, he said,  not only because of terror attacks, but also mass shootings like the one that killed 26 children and educators in Newtown, Connecticut, last December.

“So I think it's kind of dawning on Americans that we live in a fragile and dangerous world and that even our fellow citizens may come to our schoolhouse and shoot up our children.”

That realization can work against terrorist groups, according to international affairs professor Stephen Walt.

“The good news is that, provided none of these groups get access to really destructive weapons, these are not going to pose a threat to most Americans," noted Walt, "to the American way of life, to our ability to have a free and open society particularly if we don't overreact to them.”

Tracy Shea says she'll go back to the marathon next year, so as not to surrender to fear.

“If you do that, I think they won in some way and I think that's not how we live our lives here,” she said.

It's a sentiment heard a lot in Boston these days, a city already known for its resilience and grit.