https://www.thedrive.com/the-war-zone/sonic-boom-spooks-washington-d-c-area

Chasing after a Citation Jet that had gone unresponsive.

could be

What's really strange, though, is the course reversal over New York in the vicinity of its destination airport. What FMS scenario would play out like that?

Someone in the GD thread had a great theory. If followed the programmed course to line up with the runway(24 IIRC) then stayed on that route until it ran out of fuel.

ETA

https://www.ar15.com/forums/General/Plane-Crash-in-Staunton-VA/5-2649879/?page=5#i104413007

I don't know what was in the accident aircraft, but Universal and Collins would both keep flying the same heading after the last waypoint was reached or a gap if you did nothing. If you look at the route "PSK GVE SIE BRIGS Q439 SARDI CCC", if you line up CCC and KISP you can draw a line to the accident site. I would guess that was the route that was in the box.

Took a closer look at the chart, etc. Certainly makes sense that the FMS took them to Islip, turned to runway heading, and then just kept on truckin'.

Any of you jet pilots want to venture a guess if the single pilot was violating the most winked at rule in aviation, 14CFR135.89?

Can you do single pilot 135 in a Citation?  Serious question, I don’t know much about 135.

Good point. It could have been a Part 91 flight. Part 91 has similar requirements: 14CFR91.211.

It's tough to tell from the internet flight tracking data. The aircraft might have been below FL350, but not far below it.

Intercept was a day late and dollar short. Over the top of DC at 1903UTC, intercept did not occur until 1920UTC, just a few minutes before it crashed.

And here's Blanco...

I worked atc in the national capital area for 24 years. If they felt the aircraft were truly a threat to DC, the scramble would have occurred earlier.

Let me put it this way:

Post 9/11, if someone farts in an aircraft, NORAD (and everyone else) knows about it and is watching very closely.

I fly a Citation for work, though a different variant than this one.  I think someone else nailed it.  The reversal near the destination airport means the plane most likely followed the FMS plan until it ran out of waypoints.  With the Honeywell autopilot, once the autopilot runs out of points in "NAV" mode, it switches to "ROLL" mode.  It won't hold a heading, it just keeps the wings level.  Eventually they ran out of gas.  Explosive decompression is the most likely scenario.  I presume the pressurization system is fairly similar across the Citation variants, though I don't know for sure.  I can say that in the variant I fly, if there's a slow decompression you'll know about it.  Once the cabin reaches a pressure altitude of around 8000 feet you'll get a beep, a flashing yellow caution light, and an amber CAS message "CABIN ALTITUDE."  If the cabin gets above 10,000, you get the same in red, so an explosive decompression makes more sense.  The plane was cruising at 34,000 feet, per flight aware.  According to the "Pilots Handbook of Aeronautical Knowledge" at 35k your time of useful consciousness is between 30 and 60 seconds. Keep in mind this is an edition from 2008 so that may be revised by now.  If they had a slow leak, became hypoxic and unable to respond correctly, then had a blowout it's more than feasible he couldn't get to his mask.

I concur. The reason I think it was not a medical issue is that in those cases in the past, one of the passengers may have been waving to any other aircraft that intercepted them. I'm not sure if the F16's actually caught up to them and could get a look into the cabin windows or not. Hopefully we will find out the cause sometime soon.

I recall a bad door seal being written up but declined shortly before these flights.
This aircraft and it's MX/records were spotty at best.