Actually, it disintegrated, in two quick stages. The first stage was the section of the building from the top down to where the plane hit (around the 75th floor or so): it disintegrated, into shards, shrapnel, and dust. I said, "Stop!". There was a pause of a second or two, then the remaining part of the building that I could see also started to go. It took just a few seconds.
For a few more seconds, before the black clouds of smoke and pale beige clouds of dust closed in, I was looking at clear blue sky where the WTC had stood for 25 years. Then the clouds started rolling toward me.
I was standing in a corner office on the 18th floor of a building due east of the WTC, two blocks away, with an unobstructed view of the top 70 - 80 floors of both towers. (On the southeast corner of John & Nassau Sts. - address 33 Maiden Lane, to be exact.)
The beige clouds rolled over, across the WTC Plaza, across Church St., then Broadway, then to the base of the building where I was standing, then up and across all the windows, obliterating all visibility. It was at this point that someone announced that our building was being evacuated.
We started down the C stairs, but about 3 floors down, there was a lot of smoke, so we went back up to the 18th floor and regrouped. Then a rescue worker led us to the B stairs, where things were much better. It took a while to get down, as we paused several times. I work for the Federal Reserve Bank of NY, across the street from the bank's main building. It turned out that that was where we were being evacuated to, but we didn't realize it until we were out on the street. I thought we would just head away from the smoke and dust, but there was too much. Visibility was about 50 feet; the streets were covered with the beige dust. It was like walking on the moon. Many people had been smart enought to bring tissues, etc., but not me. I just covered my face with the sleeve of my shirt as we crossed the street.
At the entrance to the main building there were men with full automatic weapons, checking IDs as they announced no one was getting in without an ID. I had my ID. They led us down to the first or second sub-basement - the building has 5 sub-basements: A, B, C, D, E. There is some humongous quantity of gold on E-level, the lowest - $100 billion worth? - but I didn't get down there to contemplate the gold. The people working in the main building had already filled E, D, and C-levels.
(Note : our daughter Laura recently salvaged this email from her files. I will post part 2 if and when she recovers it -- Sue)