DANVERS ? An investigation into last November’s massive explosion at a Danvers chemical factory has determined that Heptane vapors ignited by a heat source caused the blast that damaged 250 homes, 20 businesses, 65 boats, approximately 300 motor vehicles and a school.The findings, released Monday by state Fire Marshal Stephen Coan, were similar to an initial assessment made by fire officials at the scene of the Nov. 22, 2006 incident at 128 Rear Water St. near the Danvers River.Natural gas was eliminated as a possible cause after investigators learned no such utility was connected to the building. Further, there were no indications that natural gas had entered area sewer lines or other conduits.Area residents did not report smelling any odors prior to the explosion. No natural gas vapors were found in the vicinity after the fire. Although delivery trucks transporting propane were being loaded at nearby Eastern Propane, also on Water Street, the vapors wafting over the neighborhood carried on the wind were not significant enough to cause the blast.Furnaces and dust collection systems inside the manufacturing building were working properly, eliminating these as potential sources. The U.S. Environmental Protection Agency (EPA) found no leaks in underground fuel tanks. Besides, gasses stored in the underground tanks were too heavy to explode and there was no damage to connected pipes.Through a process of elimination, investigators found that the ink processing area shared by CAJ and Arnel, the two companies in the building a 128 Water Street Rear, involves the heating of a chemical mixture. A large mixing vessel is used in the process. Witness interviews led investigators to conclude that one of the chemicals involved, about 1,000 gallons of Heptane, was most likely overheated due to a failure to turn off the steam heat system.The characteristics of Heptane make it a highly flammable, colorless solvent often used in the production of inks. Since Heptane vapors are about 3.5 times heavier than air, they tend to sink to the lowest spot in the building and can collect over time.Investigators acknowledge that while Heptane was the fuel for the explosion, the ignition source could not be specifically determined. The latter could have been a refrigerator motor or condenser, a vending machine motor, space heater, electrical exhaust fan, or the furnace.One thing is clear n the accumulation of Heptane vapors set the stage for disaster and a chain reaction occurred.The fire marshal’s report recommends implementation of a new inspection program for businesses that use or store large quantities of chemicals. Joint inspection teams are being assigned.The explosion caused damage in four communities n Danvers, Salem, Peabody and Beverly. Sixteen homes and six businesses were ordered demolished. The blast wave was felt 25 miles away. Debris was recovered a quarter mile from the river. Twenty area residents received hospital treatment for various injuries and illnesses related to the incident. Many more were evacuated.The investigation was conducted by the Fire Department, Police Department, state Fire Marshal’s Office, the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, the Essex County District Attorney’s Office, the state Department of Environmental Protection, the EPA, the U.S. Occupational, Safety and Health Agency, and other state and federal agencies.