Here's a link to a current navy, Italian lightweight sailor's cap -- wish they showed more side view of these caps.
http://www.rmr-hats.com/Sailor-Cap-109-4-0.html
Goodaim - My initial impression was that it was a Navy person. I also suspect that it could be foreign military. Could Jennifer have met a young man out one night? Could he have been saying "goodbye" as his ship was leaving? Could something have gone wrong?
I just think that, in order to get control of Jennifer without anyone taking notice, she might have known the suspect. Once they got to a secluded place, he overpowered her.
I also think that he has something around at least his left pants leg (like a band). Perhaps, he had a bicycle somewhere (as someone suggested, locked to the stairwell). Could he have biked over to Jennifer's place and caught her offguard as she was heading to work? Maybe, they were going to go somewhere to talk?
It has to be someone who wasn't missed anywhere the next morning. That could be a military person who wasn't due back to their ship/station until afternoon. That could be a college student.
I also thought of someone from work. I don't know her line of business; however, if there are employees at her office that are supposed to be at units or who are not due into work until later, that would also be a viable option to consider.
ETA: Now, that I think about it, it might make sense that a foreign military person or some person there temporarily might choose a bike as his means of transportation.