As you know it is so difficult for law enforcement to protect against these events what are called soft targets and ABC's David -- tells us more. The bloody -- explosion shattered the marathon and... See More
As you know it is so difficult for law enforcement to protect against these events what are called soft targets and ABC's David -- tells us more. The bloody -- explosion shattered the marathon and reopen wounds in this country. This attack far from the first targeting innocent civilians. International terrorists tried to bring down the World Trade Center in 93. The buildings stood the last six were killed. And domestic terrorists have proved deadly as well the Oklahoma City bombing in 95 killing a 168. And so similar to today's Boston blast the Atlanta Olympics bombing in 96. Spectators gathered -- -- park when the bomb went off it is the soft target terrorists are looking for. Unfortunately. These kinds of -- -- -- whether it's a marathon football baseball social event concert. -- -- to -- just can't get around Tom Ridge was our country's first secretary of Homeland Security we have to manage the risk. And -- have specific information about a particular actor. You can't. -- guarantee safety anyway. So even though police -- on duty in force in Boston with plenty of National Guard soldiers Stillman was able to place two bombs. Giving so many second thoughts about being around large crowds. What -- we do if we're at a larger -- is there anything that I can do to keep myself safe if you see something as a slogan is save some. It's about being aware of your environment looks unusual -- something about it sounds like your best advice is once again don't let them affect your life. About live your life right. Then that's when you when you compromise how you live. It gives them the victory and as painful as it is today. We can't let them succeed by us changing the -- changing how we it was it was delivered to us. David -- ABC news Washington.
This transcript has been automatically generated and may not be 100% accurate.