A commonly used exploding target was involved in setting off the 46,000-acre Sawmill Fire on state trust land south of Tucson, the Star has confirmed.

The off-duty Border Patrol agent whose recreational shooting sparked the fire apparently was shooting at an exploding target, when target shooting isn’t allowed on state lands, a law enforcement source confirmed late this week. In general, discharge of firearms is only allowed “pursuant to lawful and licensed hunting,” a State Land Department website says.

Land Department officials didn’t respond to several requests from the Star on whether exploding targets, which are typically detonated by bullets, are specifically banned on their lands. They are banned on U.S. Forest Service and Bureau of Land Management lands in Arizona and other Western states. But the Land Department’s websites say nothing specific about exploding targets.

The Sawmill blaze, which has cost at least $5 million to fight, spread quickly because of high winds after starting in late morning April 23 on state-owned Santa Rita Ranch, east of Madera Canyon near the Santa Rita Mountains.

At the time, winds were gusting up to 40 miles an hour and the National Weather Service had issued a fire watch, said Chuck Wunder, chief of the Green Valley Fire Department, first to respond to the blaze. A fire watch is a red flag, warning that “conditions are ideal for wildland fire combustion and that there is potential for rapid spread,” said Wunder.

His department received a fire call at 10:58 a.m. When the department’s first firefighting unit arrived at 11:11 a.m., the fire was heading north and east and already covered 300 acres, said Wunder. His family has the grazing permit for the Santa Rita Ranch but he was working, away from his house there, when the fire began, Wunder said.

Before the fire was largely contained a week later, it had jumped over the Santa Ritas and crossed Arizona 83 to attack the historic Empire Ranch and the surrounding 42,000-acre Las Cienegas National Conservation Area. Although the fire didn’t damage any buildings, it prompted authorities to put hundreds of neighboring homeowners on pre-evacuation notice. It put scores of homes, ranches and outbuildings, communications facilities and power lines at risk for several days and temporarily closed a section of Arizona 83.

The Forest Service is conducting an investigation of the blaze. It and other agencies have been reluctant to discuss how it started during the probe. The Forest Service has refused a Freedom of Information Act request to turn over records on the blaze to the Star. The Pima County Sheriff’s Department has said its records must first go through a legal review before being released. The State Land Department has not formally responded to the newspaper’s records request.

The Forest Service has also said little about the origin of a much smaller April 4 brush fire that burned 50 acres on Mount Lemmon and briefly closed a seven-mile stretch of the Mount Lemmon Highway. It has said only that it believes the fire was related to recreational shooting, citing witness reports and activity associated with where the fire started. The Forest Service said it is looking for a “person of interest” who may have started the fire. In contrast, the off-duty Border Patrol officer linked to the Sawmill Fire reported that blaze to authorities, the Border Patrol has said.

The exploding target that triggered the Sawmill Fire contained Tannerite, a compound of ammonium nitrate and aluminum powder, a law enforcement source told the Star. Tannerite is known as a binary explosive, in which its two substances are inert by themselves but can explode when mixed. Made by a company in Pleasant Hill, Oregon, it is commonly employed across the country for target practice and other purposes.

“When shot with a high-power rifle it produces a water vapor and a thunderous boom,” says the website of Tannerite Sports LLC, the Pleasant Hill, Oregon-based company that manufacturers Tannerite.

When the two substances contained in Tannerite are kept separately, they are inert and their use isn’t regulated. But when they’re mixed, the final product is regulated by the U.S. Bureau of Alcohol, Tobacco and Firearms. People can’t transport the mixed product without getting the proper permits or licenses from ATF.

The link between exploding target shooting and fire has prompted widespread concern across the West. Forest Service officials have blamed the use of exploding targets on more than a dozen wildfires across the West in recent years, according to press reports.

Contact reporter Tony Davis at tdavis@tucson.com or 806-7746. On Twitter@tonydavis987