On September 11, 2001 I was working as a programmer doing installation, configuration and tech support for Computer-Aided Dispatch systems used in Public Safety Answering Points. In common English I was a programmer working on 911 systems.
Agencies that used that company's CAD systems were among the first responders at all three impact sites. As their tech support we had more access to their records than 99% of the officers, firemen and dispatchers did. And in the days afterwards we read a lot of them researching how to improve our systems. I don't need to watch horror movies, the dry, precise recording of facts in those records is something I will remember for the rest of my life. (And it wasn't just people at the impact sites who died that day as a result of that terrorist attack.)
I spent 9/11/01 doing two things. First was helping agencies in my area of responsibility deal with minor glitches as they opened up their Emergency Operations Centers. (There weren't many, these are people who take their jobs seriously and plan for contingencies.) The second was staring at the grey cloth-covered wall of my cubicle waiting to hear that one of our clients had gone off the air. Fortunately no 911 center was a target but I can still clearly see that grey fabric.
Sorry, that's probably too much information. My thoughts and sympathies also are with the familes of all but 19 of the dead.