Fire investigators say they can’t figure out what sparked a fire at an apartment in Bangor on Thursday.Crews were called to the structure on Third Street around one in the morning. We’re told the fire started in a second floor bedroom of one apartment, but it’s not known how.The fire displaced five people who lived in that apartment unit. There was smoke damage to the rest of the building.No one was hurt. Crews were called to a second fire on Third Street later in the afternoon. Investigators say the soldering of frozen pipes caused that fire.