Why Rim Fire’s alleged igniter will not face charges

Inmate firefighters walk along state Highway 120 as firefighters continue to battle the Rim Fire near Yosemite National Park, Calif., on Sunday, Aug. 25, 2013. Fire crews are clearing brush and setting sprinklers to protect two groves of giant sequoias as a massive week-old wildfire rages along the remote northwest edge of Yosemite National Park. less Inmate firefighters walk along state Highway 120 as firefighters continue to battle the Rim Fire near Yosemite National Park, Calif., on Sunday, Aug. 25, 2013. Fire crews are clearing brush and setting ... more Photo: Jae C. Hong, Associated Press Photo: Jae C. Hong, Associated Press Image 1 of / 43 Caption Close Why Rim Fire’s alleged igniter will not face charges 1 / 43 Back to Gallery

Federal prosecutors said Friday that they intend to drop charges against the bow hunter accused of starting the devastating Rim Fire that burned through Yosemite two years ago and convulsed into the largest wildfire in Sierra history.

In a motion to dismiss the case, U.S. Attorney Benjamin Wagner said the two witnesses who needed to show that Keith Emerald, 33, of Columbia (Tuolume County), ignited the blaze by making an illegal campfire had died.

One person who spoke with Emerald shortly after Emerald was rescued from the fast-moving fire via helicopter died in a workplace accident in February, according to prosecutors. The second witness, the helicopter pilot, died last month of cardiac arrest, prosecutors said.

Statements by the two during the investigation are now considered hearsay and cannot be used as evidence at a trial.

“I appreciate the hard work done by the U.S. Forest Service in investigating this case, and I understand that the government’s motion to dismiss will be frustrating to some,” Wagner said in a press statement. “However, when circumstances change after indictment, and our judgment is that a case is no longer likely to be proven beyond a reasonable doubt, it is our obligation to the defendant and to the court to dismiss that case.”

The petition to drop the charges must be signed off by a judge at the U.S. District Court in Fresno.

Emerald was charged in August with a felony count of setting fire to a forest as well as misdemeanor counts of leaving a fire unattended and violating local fire restrictions. He was also charged with one felony count of initially lying to investigators about the cause of the blaze.

Prosecutors had maintained that Emerald was on a solo hunting trip in the Stanislaus National Forest on Aug. 17, 2013, when he lit a campfire to heat soup, and winds picked up and carried flames into tinder-dry brush.

Campfires were prohibited at the time in that area, along the Clavey River canyon.

The fire rapidly spread beyond Emerald’s control, prosecutors said, and went on to char 400 square miles, including the western edge of Yosemite National Park.

The fire burned for more than two months, shutting park roads and prompting visitors to cancel vacations during the height of the summer season. The blaze destroyed 11 homes and dozens of other buildings, including the city of Berkeley’s Tuolumne Camp.

Kurtis Alexander is a San Francisco Chronicle staff writer. E-mail: kalexander@sfchronicle.com Twitter: @kurtisalexander