Most likely not a prank/lark. The last arson was perpetrated by a guy who was trying to destroy criminal evidence against him.
This fire started in the sheriff's office, which would definitely be cause for suspicion. Only time will tell.
Unfortunately, if it was someone trying to destroy evidence, they would have to be quite cagey, since a lot of current evidence/information is shared with other law enforcement agencies, etc. That means that a great deal of the facts/information on a crime are still accessible. Oh, well. There's always room for stupid crooks.
"As a girl I had zero interest in the stove." - Julia Child
This is real inspiration. Look what Julia became!