Yes it is a jail. I have just posted a video with the audio tour of the jail. It has some amazing history, my video goes for almost 1 hour, if you ever have time you can skip the start of the video and just go to the recorded audio and hear the stories from men that worked there and some of the prisoners that did time there. I really find it amazing and just imagine what it was like back then.
https://steemit.com/alcatraz/@windwalker/alcatraz-full-tour-with-audio