A two-alarm fire damaged a vacant duplex unit Saturday morning in north Modesto, and investigators believe the person intentionally lit the blaze, fire officials said. No injuries were reported. The fire occurred about 7 a.m. at the home in the 1400 block of East Rumble Road, just east of Sylvan Elementary School and Coffee Road. Residents in the duplex's adjoining home got out safely, said Modesto Fire Battalion Chief Hugo Patino. He said the fire started outside the home near a door that leads into a small utility room. The flames made their way inside the home and reached the attic. Firefighters had the blaze under control within 20 minutes. Patino declined to say what evidence led investigators to believe the fire was arson, fearing it would harm the investigation. The fire caused an estimated $25,000 in damage. The fire damage was limited to the vacant unit. The residents in the other home were not displaced.