My understanding is they're focusing on everyone who was in the building that day. And if someone had a motive to start a fire and wanted to make it appear that Mark Anderson had started it, around his area is exactly where they'd do it. Mark Anderson had no motive to start the fire and is not responsible for it.

I've tried to set up a meeting with them and haven't heard back. So, technically, the ball's in their court.

He would have no motive whatsoever to set a fire. Mark may be the obvious culprit, as in 'the butler did it.' But I've played enough Clue to know that the butler never does it.