It's federal IT. I'm going to guess overload due to extremely resource-inefficient implementation before I guess "uh oh, somebody's curious about MKULTRA, better pull the search".

I haven't gotten the search to work at all, assuming that search bar is for the documents (the UX isn't great). I've been getting "CIA.gov is Temporarily Unavailable" each time.

That said, there are some interesting (and mildly terrifying) documents you can find from some general browsing of the categories.