MOUNT VERNON — Investigators have determined the fire that burned a large vacant building Sunday in Mount Vernon was set, but they don’t know whether it was intentional or an accident.Police said the fire started in an old office on the second floor of the Alf Christianson seed building. It has been vacant since 2011. Damage is estimated at $500,000.Police arrested a transient on the roof as the building burned. He said he was awakened by the crackling sound of the fire. He was released.The building was searched Monday with a cadaver dog to make certain no other transients had been trapped inside. No bodies were found.