When a fire ripped through an old house on the south side of Magnolia in May, no one thought much of it.

Less than 24 hours later, a squat, white house around the corner lit up in the night, sending firefighters again to the neighborhood dotted with shotgun houses and overgrown lots.

By last week, the seventh suspicious fire had broken out in the area, confirming to Magnolia Fire Chief Greg Pinner that he has trouble.

“Something’s obviously going on,” Pinner said last week after the latest fire leveled a vacant house in the city’s poorest neighborhood. “W...