Where did the cockroaches come from, what attracted them, and will there be anymore?

yeah so basically a few weeks ago we moved into a condo. We do have neighbors on the top, and bottom floor. I don't think we have cracks or holes in the walls of our condos, but we do have air vents. so a few days later I wake up, my dad also wakes up, i walk into the kitchen, and all of a sudden I see this big, flat rounded bug dart from behind our stove, and run into the sink, and then it just stays there. So my dad sees the cockroach, and kills it then throws it out. Then a week later I see another one in the bathroom, just sitting their not moving. The next day I look in the bathroom, and it's gone. So another week later I'm watching TV, and I see another one darting behind the TV going to the kitchen. Now this one was a small one. When it went in the kitchen it ran into a crevice, and I never saw it again. But after that I never saw another one again. The only places in which I saw them were in the bathroom, and in the kitchen. Can somebody tell me why there weren't any in my room, and will there be more?