Mourners gathered in Lower Manhattan for a special ceremony marking 15 years since the September 11th terror attacks. NY1's Roger Clark filed the following report.

A bell marks the exact moment that started a chain of events that changed the city and world forever - when a plane struck the North Tower of the World Trade Center.

The city marked 15 years since the terrorist attacks very much like it has each and every year since - the reading of the names of the nearly 3,000 who perished on that day and in the 1993 Trade Center bombing. No politics, no speeches, just a remembrance of family, friends, and co-workers who are no longer with us.

"It takes 4, 4 and half hours to hear all of those names, and we owe it to every one of those people to listen for their name," says Alice Greenwald, Museum Director at National September 11th Memorial and Museum.

There were six moments of silence in all, remembering when the planes hijacked by terrorists hit the towers, when the towers fell, and when a plane slammed into the Pentagon, another crashing in a field near Shanksville, Pennsylvania. Memories of those lost - the backdrop of Lower Manhattan that continues to bounce back from the tragedy.

"On occasions like this, we've got to mourn, we've got to remember those who are lost, we've got to remember our fallen heroes," Homeland Security Secretary Jeh Johnson said. "But we've got to keep rebuilding."

And all these years later, those impacted by the attacks and others charged with helping to remember say it's even more pivotal to educate the world about what happened that day 15 years ago.

"We want to make sure that we never forget because we don't want it to happen again," said Frank Siller, who lost a brother on 9/11. "We don't want 3,000 more families to go through the same thing."

"I think there is even a sense of greater urgency in some respects to focus, to do the commemoration in a meaningful way, for families, for those who lost loved ones," Greenwald said.

She says there is a vigorous commitment now to education, because there is a new generation that needs to know what happened on that day.