For most of the world including myself, it was United Flight 175 hitting the South Tower that made it clear that what was happening was not a “mere” accident. Two planes hitting the World Trade Center could not be a coincidence. From the hour that followed the second crash, I will always remember Gerry Callahan reporting, in a clear and restrained voice, that the South Tower had completely collapsed. If any doubts had lingered in the immediate aftermath of the second crash, there were now none that something irreversible was happening.
The first people to be irreversibly impacted, of course, had been the passengers and crew on American Flight 11 and the people immediately killed in the North Tower, then those forced to jump to their deaths by the resulting fire. With the second crash and the eventual collapse of both towers, the killing would expand to the passengers and crew of United Flight 175 and the immediate and near-immediate South Tower victims, then to the people trapped in both towers, most of them above the crash-sites unable to get down before the collapses, and to the police and fire personnel from New York City who went into the towers to rescue anyone who was in there. I think I remember some very early reports that gave estimates of as many as 20,000 potential deaths in New York City alone.
Miraculously, the numbers did not reach that high. 2,753 people were killed in the attack on New York, 184 were killed in the attack on the Pentagon, and 40 people were killed on United Flight 93 on September 11, 2001.
Fortunately many were not at work yet. There were also many unsung heroes that day that assisted others to evacuate, not just firefighters, EMS & police.
Also, most FDNY members would rather think of that day as the greatest rescue effort in the history of the FDNY as opposed to their darkest day!