Hi Karsten - thank you very much for your help. The record where I originally found the information definitely states that he was living in "Tugthuset" in 1866, so, unless the exact reference wasn't correct, he must have been in one of the two prisons you mention. This will be a big help. I believe he was probably released just before 1870, so the census records may not give me any information, but now that I have the likely prison locations, I will try contacting the Danish Archives and see what is available. Since he lived close to Copenhagen, I am assuming he probably would have been at Vridsloeselille. Thank you again for the help!