It’s hard to fathom that teens now entering college — or serving in the armed forces — were not yet born on that unforgettable Tuesday morning. Nearly 3,000 people died in Manhattan, at the Pentagon and in a Pennsylvania field on September 11, 2001, in what remains the deadliest act of terrorism in history. To mark the 20th anniversary of that infamous day, we asked people with a connection to the attacks to reflect on what they experienced then and what it means to them today. Bill Keegan, a lieutenant in the Port Authority Police Department:The World Trade Center in lower Manhattan was a beacon to the whole world. It said, “We want to know who you are, we want to work with you, and we want to trade with you.” Trade allows us to understand different cultures and different people, and for them to understand us. I think that’s why these terrorists targeted the towers.