Milwaukee fire investigators say they can’t rule out arson as the cause of a weekend fire that  displaced 15 people late Saturday and early Sunday morning.  Witnesses say the flames were first seen at about 11:30 P-M.  The fire apparently started in the back of the building, traveled up the back wall and entered the structure through the roof.  The south-side neighborhood has had similar fires.  People living nearby say trash cans were set on fire in an alley last summer.  Arson is a strong possibility because the problem started outside the house.