( CNN ) -- A faulty monitoring system on a gasoline tank at a Puerto Rico storage facility most likely led to a massive fire in October that damaged homes and businesses more than a mile away , federal investigators said Tuesday . A storage tank at the Caribbean Petroleum Corp. facility in Bayamon , near San Juan , was being filled with gasoline from a ship docked in San Juan harbor when the tank apparently overflowed , U.S. Chemical Safety Board investigators said at a news conference . `` Gasoline spilled from the tank without detection ; as the material spilled , it vaporized and spread across the facility , '' the agency said in a release . `` CSB investigators estimate that the vapor cloud spread to a 2,000-foot diameter until it reached an ignition source in the northwest section of the facility . '' The liquid level in the tank could not be determined because the facility 's computerized monitoring system was not fully operational , the CSB investigators said . To monitor the level in the tank , operators used a mechanical gauge on the tank 's exterior wall , the investigators said . As the gasoline level in the tank rose and eventually overflowed , employees in the facility 's control room were not aware of the situation . `` The filling of a tank without a functioning monitoring system is the type of activity the CSB will be examining very closely , '' said lead investigator Jeffrey Wanko . `` The CSB 's investigation will examine operations particular to Caribbean Petroleum , but will also look at the regulations and best practices surrounding the industry as a whole in an effort to improve safety practices at similar facilities . '' The fire , which started at 12:23 a.m. on October 23 , burned for three days . There were no fatalities , but thousands of residents were forced to evacuate to avoid the thick clouds of black smoke . The FBI has determined that the explosion was an industrial accident , said Agent Harry Rodriguez of the San Juan office . `` It was not a terrorist act or an act of sabotage , '' he said . He said the FBI had determined that graffiti found on two San Juan tunnels that referred to a fire were not painted by anyone with connection to the blaze . The spray-painted message on the tunnels , less than three miles apart , said : `` Boom , fire , RIP , Gulf , Soul , ACNF . '' Caribbean Petroleum owns the Gulf Oil brand in Puerto Rico . Nonetheless , Rodriguez said , the FBI and the Bureau of Alcohol , Tobacco , Firearms and Explosives were still investigating . Caribbean Petroleum supplies a significant portion of Puerto Rico 's petroleum products . The facility includes a tank farm and refinery that was shut down in 2000 . Before the fire , the tank farm stored gasoline , diesel fuel , jet fuel and fuel oil in about 30 aboveground tanks . The Chemical Safety Board is an independent federal agency charged with investigating industrial chemical accidents . CSB investigators look into all aspects of chemical accidents , including physical causes such as equipment failure , as well as inadequacies in regulations , industry standards and safety management systems . The board , which is appointed by the president and confirmed by the Senate , does not issue citations or fines . The board does make safety recommendations to plants , industry organizations , labor groups and regulatory agencies such as the Occupational Safety and Health Administration and the Environmental Protection Agency . CNN 's Arthur Brice contributed to this report .
Oil tank fire at the Caribbean Petroleum Corp. has been investigated . Tank exploded , creating vapor cloud that ignited , officials said . FBI says the fire was an industrial accident , not terrorism or sabotage .