When the smoke had begun to clear, and when day had begun to emerge from the dark, a scene of unimaginable desolation emerged. Where two of New York's tallest buildings stood, there was now rubble: a pile of twisted debris so huge that it nearly rivaled in size some of the buildings around it that remained standing.
Then the next horrific question: Would the collapse destabilize surrounding buildings? Emergency personnel and onlookers would have an answer by 5:20 p.m., when 7 World Trade Center - a 47-story tower that hours earlier lay in the shadow of its giant neighbors - collapsed.
A crowd of 100 to 150 onlookers, media and emergency personnel standing five blocks north of 7 World Trade Center gasped in horror when it fell in another hail of debris and dark gray dust, in contrast to the black, oily smoke that had poured from the Twin Towers debris pile.
Just a half-hour before the collapse, at 4:47, Con Edison had shut down a power substation in the building's basement, effectively shutting off power to all of lower Manhattan.
Law enforcement sources told Newsday after the building's collapse that some Secret Service agents and agents of other federal investigative forces were unaccounted for and may have been in the building when it came down.
Emergency personnel - aware the unstable building, also known as Tishman Center, could give way at any time - had earlier cleared the area around the building, north across Vesey Street from Tower One of the World Trade Center.
The building was in flames by 12:30 p.m. By 5 p.m., it became clear that this building was doomed, too.
Just before 7 World Trade Center collapsed, it was engulfed in a hirlwind of flames, smoke and dust. The heat created a mini-cyclone that sucked paper out of windows from nearby office buildings.
As the building collapsed, some people in the group at Church and Reade streets began running west, then stopped and returned to watch as the debris settled. Behind gas masks, tears streamed down the faces of some exhausted firefighters.
"First thing, you just saw all the windows were breaking. Then it just pretty much went down," said Kirk Wilson, 23, who works in the financial district and had traveled from his home on Houston Street yesterday afternoon to see what he could. "It wasn't loud at all."
When the smoke cleared, a pile of rubble about five stories tall was all that remained. By 5:40 p.m., firefighters moved in to extinguish fires that had erupted in vehicles nearby and to train hoses on the building rubble.
Shortly after 6 p.m., emergency personnel again began heading toward the mountains of debris from the Twin Towers' collapse to continue efforts to find survivors.
Just after 7 World Trade Center collapsed, police moved the crowd of onlookers a block farther north to Duane Street.
"If you don't need to be here, LEAVE!" shouted one supervisory official, who was in plainclothes. "THIS IS NOT A MOVIE!"
Staff writers Mae M. Cheng, Patricia Hurtado and Jessica Kowal and freelance writer Leonard Post contributed to this story.