That was the moment, 18 years ago, when the North Tower of the World Trade Center collapsed.
A procession of firefighters, police officers and first responders then marched from Fireman's Hall Museum in Old City to the Betsy Ross House for a bell-ringing ceremony.
Anthony Boyle, Drum Major for the Philadelphia Police and Fire Pipes and Drum, says ceremonies like these are not just to remind us of the attacks, but also how our country united in the aftermath.
He says, "Just how important is it for us to remember and realize how we were as a nation back on 9/11 and that feeling we need to capture again."
Local leaders and dignitaries spoke during the emotional ceremony.
Among them was Blanche Carney, Commissioner of the Department Of Prisons in Philadelphia.
She says 18 years has proven insufficient to bury the emotions that remain from that terrible day.
Holding back tears she said, "Our brothers and sisters got up that very morning, put on their uniforms, and went about like any other day unaware of the tragedy that would strike."
And as they rang a bell in honor of those who died, the hope is to make sure that the first responders who ran in to help will never be forgotten.
Philadelphia Managing Director, Brian Abernathy, says, "First responders are really are amazing people and we should feel blessed and be thankful for what they did."