While law enforcement officials continue to investigate the Boston Marathon attack, other cities are reassessing their vulnerabilities.

America's older cities always face a threat, said former Philadelphia Police Commissioner John Timoney.  "Philadelphia has some of the most historic buildings and iconic structures of any city in America," he said. "Philadelphia, Boston, New York are really the three oldest cities that have these historic iconic buildings that are always -- there's always a threat."

A police presence, security measures and cameras can help protect some spots, Timoney said. But he said big events out in the open are challenging.

"Whether it's a regatta along the Schuylkill River, whether it's the July Fourth celebrations, whether it's the Broad Street Run or the Philly Marathon, those are where you're very vulnerable because they cover a long space as far as miles are concerned, No. 1," he said. "No. 2, there's no way of controlling the flow of people." 

In those cases, human intelligence can be essential, he said.

Allowing visitors to easily stop by Independence Hall, the Betsy Ross House and other historic sites is important, said Jack Thomas Tomarchio, former Principal Deputy Under Secretary for Homeland Security for Intelligence, but it can mean living with an increased vulnerability to an attack. 

It's very challenging to try to secure an event such as the Broad Street Run or the Philadelphia Marathon, said Tomarchio, who now owns the Agoge Group in Wayne. 

"Certainly these are what we would call soft targets. And it's very difficult to secure a shopping center or a sporting event, a high school football game where the idea is people are out mingling having a good time," he said. "This may, unfortunately, be the next generation of a terrorist attack."

Tomarchio's advice?  If you see something, say something.  

Keeping attacks in perspective

The country must keep going, despite what unfolded in Boston, says Ian Lustick, a University of Pennsylvania professor of political science.

"I think it does point out, ironically, how easy it is to attack targets in the United States in these ways and how remarkable it is that we haven't had more of them," he said.

Lustick said while the incident does raise important questions about security for public events, "you can't have 100 percent security.

"In some ways, the country would be better off -- despite the horrors -- if it doesn't portray these attacks in an ongoing dramatic way.  We have to learn to absorb a certain amount of horror in our lives in order not to encourage copycat activities," he said.

Lustick said treating every terrorist attack as something that threatens our existence will only encourage more of it.