The terrorist attack near the Boston Marathon finish line was in a 'soft' target area.
Although security was tight and numerous police were present, the area was vulnerable and large crowds could wander freely.
Roger Cressey is a counter terrorism expert who served under presidents Bill Clinton and George W. Bush.
He also ran the Boston Marathon in 2010 and 2011.
Cressey admits when he was running in Boston "my security was the last thing I thought of. I was exhausted and exhilarated at the same time."
He speaks to Marco Werman about the challenges of securing public areas and lessons learned since September 11th.