The cause of the fire at the long-vacant commercial structure on Nevada City Highway in Grass Valley, that occurred early Sunday morning, is still undetermined. But there are now a few more details. A press release from the Grass Valley/Nevada City Fire Department says the blaze, across from Lumberjack’s, appears to be human caused. Investigators say indicators point to it starting in the rear, or highway side, of the building, in the area of a stairwell at ground level. They also say the fire had been burning for some time prior to its detection, resulting in the building being 25-percent involved upon the first arrival of crews.