(Vancouver, WA)  --  Investigators say it will likely be a few days before they determine what caused a fire that destroyed an old pine cone factory in Vancouver.  Police are searching for three "persons of interest" in the case, believed to be around 16- or 17-years-old.  Witnesses say they saw at least one of the youths carrying what appeared to be a gasoline canister.  The fire broke out Friday at the Northeast 28th Street property, which had been sold to a California buyer who planned to develop it into apartments.