State and local fire investigators believe a fire that burned through a historic Reading home and barn in July was arson, the Department of Fire Services announced Wednesday, offering a reward for tips. “Based on our examination of the scene, witness interviews, and other evidence, we believe this fire was intentionally set,” said Reading Fire Chief Richard Nelson. “I want to be clear how serious this incident was. It went to three-plus alarms, two structures were completely destroyed, and several firefighters were treated for heat exhaustion after working for more than 12 hours to battle the flames.” Just after 1 a.m. on July 9, an off-duty firefighter reported the blaze at 186 Summer Ave., a structure consisting of a three-story, wood-framed, historic home and barn in the rear. The Reading Fire Department responded to find “heavy fire” at the site, quickly entering to attack the blaze, the department said. The firefighters were forced to exit as “conditions inside became untenable and posed grave hazards to personnel” and attacked externally to protect the nearby homes. The fire was brought under control at around 2 p.m. the next afternoon, the fire authorities said. The home was unoccupied and under construction at the time of the incident, the department said. No civilians were reportedly injured in the blaze. Twelve mutual aid communities assisted in the response, the department said. “After weeks of joint investigation with our local and state partners, we’ve now reached a turning point,” said Reading Police Chief David Clark. “We believe members of the public may have information that could help us move the case forward.” The facts and circumstances of the fire “remain under investigation,” the department said, though authorities stated the cause has been determined “suspicious.” Tips can be made to the Arson Watch Reward Program confidentially and anonymously, fire officials said. “We’re asking anyone with knowledge about this fire to share it through the Arson Watch Reward Program at 1-800-682-9229,” said State Fire Marshal Jon Davine. “This could include direct knowledge about the person or persons involved. It could include observations of people or vehicles seen on Summer Ave or near the Parker Middle School late on July 8 into July 9.” Rewards are offered by the Massachusetts Property Insurance Underwriting Association and may go up up to $5,000 “for information that solves, detects, or prevents arson crimes.”