To recall the nightmare that came screaming out of a brilliant blue sky 20 years ago is to rediscover the plain jarring truth that we never know for sure what the next moment will bring.
Even fully grasping what you just saw can be a challenge.
Today I can still conjure the giant boom and roar from across the East River of the second hijacked jet as it smashed through the World Trade Center's south tower.
"That plane flew right in! It’s a TER-rorist attack!" shouted a heavyset guy maybe 20 feet away on the promenade above the Brooklyn-Queens Expressway, head turning, eyes bulging behind thin wire-framed glasses.
We'd been standing among dozens of people, watching and wondering about the fire from the first crash, above what we later knew to be the north tower's 93rd floor. At the second impact, all possibilities of accident vanished.
Subways still ran. Downtown, sirens of countless emergency vehicles screamed as if in chorus. Outside City Hall we heard a combined groan or screech or blast or rumble — maybe all of those — as the south tower fell. People ran, covered in dust and chased by a toxic rolling earthbound cloud.
A few of us joined this crowd in flight from the horror zone, and stopped by Foley Square for a while.
"Move north! Everybody MOVE NORTH NOW," shouted a police officer. Those federal buildings could be next, the cop warned. They weren't next, but nobody knew. We moved.
After the second collapse, people wept and prayed in the first-floor chapel in the Bowery Mission on Lafayette Street.
At N. Moore Street, a staging area was cleared for those who might be injured. Stretchers never came. People lined up to use pay phones. Cell service was knocked out. One man, a stockbroker, described carnage he'd seen up close hours earlier. What would come next?
At 5:20 p.m., 7 World Trade Center, a 47-story structure weakened by all-day fires from flaming debris, collapsed about 10 blocks downtown. Another big roar followed and, after a while, we were moving north again.
Trudging uptown, a veteran news photographer I'd just met asked in his French accent: "So what will it be: Internment camps for Arabs, or Western passivity?"
In a Burger King near Sixth Avenue, I ran into a college acquaintance, Peter Johnson Jr., by then a lawyer with personal and professional connections to the city government and its uniformed services.
Having covered the FDNY, I thought the moment called for Deputy Chief Ray Downey, a nationally known rescue and recovery expert from Deer Park whose credentials included leading operations after the 1995 Oklahoma City bombing.
To my sad surprise, Johnson informed me that Downey was among the missing. Also gone were others we knew: The 71-year-old first deputy commissioner, Bill Feehan, and Chief of Department Peter Ganci, and FDNY chaplain Father Mychal Judge, who performed Johnson's wedding ceremony, and for whom Johnson would later deliver a eulogy.
Near sunset, the train back to Brooklyn clattered onto the Manhattan Bridge. As New York Harbor came into view, passengers rose from seats, moved to the side, stood and gazed out the windows, silently, at where the towers had been.
Looking back, you contrast this impromptu moment with the now-typical sight of faces buried in electronic devices.
For disasters, deep memory needs no smartphone. The tough part for even the luckiest would be making sense of what we faced.