It's everyone's worst nightmare. In an instant, the Boston Marathon turned from celebration to terror.
Security experts call the Boston Marathon a "soft target" because most people have their guards down, never expecting a terrorist attack at an event like this.
Now, there are security concerns about other upcoming soft target events, like the NBA playoffs that are about to get underway, and the Kentucky Derby in May.
Michael Sullivan, former director of the Federal Bureau of Alcohol Tobacco and Firearms told INSIDE EDITION's Les Trent, "A soft target is a place where it doesn't seem to have any type of permanent security infrastructure to it. This is something, obviously that terrorists, domestic and international look at."
In the wake of the Boston Marathon bombing, security is being beefed up around the nation, including Los Angeles' Dodger Stadium.
New York City's Times Square is considered a prime soft target because it's filled with hotels and restaurants where visitors come to have fun.
Former Secret Service agent Evy Poumpouras showed INSIDE EDITION's Les Trent steps you can take to protect yourself.
Pointing out a trash can, Poumpouras said, "We need to pay attention to what's going on around us. Here, we have a trash can. You want to not stand near any type of trash can, or mailbox or anything like that, or newspaper bins. What we want to avoid is glass. If we were to have any type of explosion, even if it's at a distance, the blast waves could shatter the glass and that could cause you severe injury."
Poumpouras continued, "When you get to your spot, asses the situation. Where am I standing? What would I do if something happens? You want to 'what if' something before anything happens. You know why? Because that give you the few seconds that you may need to save your life."
INSIDE EDITION also caught up with Celebrity Apprentice host Donald Trump, who, like everyone else, is shocked by the massacre.
"The Boston attack is a total outrage. It's disgusting and something has to be done. If they catch these perpetrators, and they must catch them, the punishment has to be very, very harsh, and has to be swift," said Trump.