They declassified a lot of it a couple of years ago, I saw a good documentary on what was going on there. They had pictures of the crash site clean-up and everything (it was an Angel or Blackbird that crashed iirc, which does agree with the reports of a strange, not seen before material)
The Roswell incident predates Angel/Blackbird by decades.
Regardless, the incident at Roswell has long since been declassified. It was a crash of flight number 4 of Project Mogul, a use of high-altitude weather balloons to detect Soviet nuclear tests.
If you read the original written reports of what was found, it describes very clearly a prototype Project Mogul payload: a circular array of microphones and radar transponders attached using sticks, rubber strands, and scotch tape, and wrapped up in aluminium foil (for EFI shielding and radar reflectance).
IIRC I think George Dyson wrote a book or article on Project Mogul at some point... or at least he researched it in depth enough to talk in detail about it for 30 minutes at a lecture of his I went to. My google-fu can't seem to find a citation to him though.
It's actually a very interesting history linked to the U-2, A-12/SR-71, etc. Seismography and atmospheric radioisotopes ended up being an easier and more precise way to detect Soviet nuclear testing. But the balloons developed were then used to spy on and map Soviet installations in Siberia, dropping their camera payloads in the sea of Japan. Of course the same sort of overflights would later be done by the U-2 and then SR-71 until the early Keyhole satellites made the very concept of atmospheric overflight obsolete.
You know, I wouldn't be surprised if the city of Roswell itself, if not the state of New Mexico, had an interest in keeping the mythology alive and the documents classified. When the Roswell incident struck a chord with the public, it probably put the city on the map to some degree. I'm sure it has driven a fair amount of tourism over the years, if not strong pop-cultural iconology.
New Mexican here. The fascination is pretty much confined to Roswell. Yes, it definitely put the city on the map, and yes, it drives some tourism. But Roswell is not large and it's hard to imagine how a city would petition for classified stuff to stay classified. Besides, cranks are cranks and no quantity of facts will serve to dissuade them. Roswell's UFO crap is already extremely hokey. It's part of the culture now, so it doesn't matter if there's a reason for it or not. Just like Zozobra up in Santa Fe, it's just this thing.
Sorry I've looked it up and it appears I was mistaken about the source of the infamous 1947 crash. The OXCART project did operate out of Area 51 though, and there were a number of crashes and cover ups - they were working on spy planes after all!
