Nearly 3,000 people were murdered on September 11th. Their names are engraved on the memorial at Ground Zero. For their families, the past decade has been full of struggle, recovery, and resilience.
CBS News correspondent Russ Mitchell recently talked with one father who embodies all those qualities every day.
Lee Ielpi is giving a tour he's led about 1,000 times.
"From point impact, 94th floor to the top, 1,365 people trapped or killed, no way out," he said while giving a tour.
But he never tires of talking about his son, Jonathan.
"Loved the work, loved firefighting, loved helping people," Lee said of Jonathan.
Lee, a Vietnam veteran who spent 26 years in the New York City Fire Department, saw both of his sons, Jonathan and Brendan, follow in his footsteps. But on 9/11, Jonathan, a 29-year-old father of two, didn't come home.
"What do you remember most about September 11th, 2001?" Mitchell asked Lee.
"My son's phone call. He said, 'Dad, we're going to World Trade Center. I said, 'Okay buddy, be careful.' He said, 'Okay, dad.' And that was the last time we spoke."
After the towers collapsed, Lee joined the search, looking for his son and his squad.
"Could they be underneath that rubble?" asked Lee. "Fingers crossed. But then thinking about it... no."
Lee spent nine months on what was called "the pile." Jonathan's body was found after three. A blessing, says Lee. More than 1,100 families -- 40 percent -- never received any remains.
He became a passionate voice in debates about what should be built on these 16 acres.
"I know that the memorial site now is because we were so adamant about preserving a good section of that site. We, the family members," Lee said.
As head of the September 11th Families Association, Lee keeps a close eye on the rebuilding from his office window. He considers it an accomplishment that half the site -- 8 acres -- is devoted to the memorial.
Since 2005, Lee and other family members have given these tours around the site and inside the "Tribute Center," a mini-museum Lee co-founded near Ground Zero. Its artifacts have been seen by 2.3 million visitors, who have left enough comment cards to fill a new book, "9/11: The World Speaks."
Holding up the book, Lee said: "They all speak with the same voice -- we must find a way to live with each other regardless of our differences. We must live in tolerance, we must find a way to stop hatred and terrorism, and we must educate and enlighten."
Lee shares his most personal memories to educate, such as son Jonathan's fire coat.
"Is it ever hard for you to come down here and do your job?" Mitchell asked Lee.
"Every tour...because I'm gonna cry. You know, it's okay to cry. Now let's use those tears to in a positive way," he said with a smile.
Lee's son, Brendan, is now in his 10th years as a New York City firefighter. He now works in the same high risk rescue unit where his dad once served.