Rebuilding the site has famously been a source of contention for everyone from politicians to developers, but it was many of the mourners who fought against rebuilding on what they called a sacred place.
"I think that's why it was so important that those people who felt immediately and understandably -- do nothing, leave this as a sacred site -- were wrong," said David Childs, the architect of One World Trade Center. "It was an honor of the people that died there, they were working there, to rebuild and say, 'Pick yourself up.' It's in our DNA to want to rebuild after disasters."
Progress has been slow since the buildings fell amid a series of political and bureaucratic wrangling. Now, the work never stops. Construction has been nearly ongoing year round.
"It never stops," Baroni said. "We can't afford to stop."
Right now, there are 3,500 construction workers on all four corners of the site. By the time the project ends, Baroni said, 25,000 different construction workers will have worked on it.
Baroni said the schedule is set by a series of deadlines -- and the next one is for the museum -- set to open in 2012. Soon the office building will be finished, and then the transportation hub, on which construction has yet to start.
The museum will showcase a collection of artifacts, photographs, personal effects and memorabilia, and expressions of tribute and remembrance.
Several oral histories are already available online.They include testimonies from a man who exited the south tower just before it collapsed, and the attorney in charge of the September 11th Victim Compensation Fund, which was created by Congress to provide economic relief to the families of those killed in the September 11, 2001 attacks, as well as to those who were physically injured that day. They include testimonies from a photojournalist who describes how she felt as she witnessed the south tower collapse, and a woman who put together a collection of drawings and letters children sent to fire departments after the attacks.
ABC News' Maggy Patrick contributed to this report