What kind of beetle is this?

So i was at my friend's house last week and when we went into this one room there were beetles...EVERYWHERE. they were mostly by the window and curtains. some where climbing up walls but it seemed like if they stayed on the walls or ceiling too long, they would fall off. they were black and maybe one inch in length. they had wings but we never saw them fly. they werent very fat, and if it helps, this is in northern Illinois.

they weren't very easy to kill since they seemed immune to raid or other bug sprays. it took multiple blows with a fly swatter to squash them. also, we thought it was weird that they isolated themselves to only one room in the house with the exception of one brave soldier who ventured out into the next room.

this is more out of curiosity than urgency since they dont seem to be there anymore.

we just want to know what they were and if they have a weakness in case we ever see them again.