Ok "swim" first hire a good fire investigator and let them look at the evidence and two advice you.

One question you said swim was in there after hours,
1. WHY????
2. Was there any one else in the building after hours????

Second sounds a little slim for evidence. Anyone could have lit the fire or dropped a cigarette prior to swim entering or exiting the room. The fire could have started in a short time frame or over a litlle longer one.

So if no one saw the person actualy light the fire than sounds like a little problem.

Yes a cigarette can be used to start an arson fire.

No have not heard time of day negating an arson charge.