FRESNO, Calif. — Criminal charges have been dropped against a hunter accused of starting a massive wildfire that burned parts of Yosemite and the Stanislaus National Forest, federal prosecutors said Friday.

The decision came after two key witnesses unexpectedly died after the indictment was handed down last year against Matthew Emerald, prosecutors said.

The 33-year-old California man was accused of starting the 2013 blaze called the Rim Fire that burned for two months, scorching 400 square miles and costing $125 million to fight.

Prosecutors say that without testimony from the two witnesses, they don't believe they could have proved the allegations beyond a reasonable doubt.

"I understand that the government's motion to dismiss will be frustrating to some," said U.S. Attorney Benjamin Wagner. "It is our obligation to the defendant and to the court to dismiss that case."

The blaze ranks as California's third-largest wildfire and the largest in the recorded history of the Sierra Nevada.

A message requesting comment from an attorney for Emerald was not immediately returned.

Authorities say one of the witnesses, who had talked with Emerald shortly after being rescued by helicopter from the wildfire, died in February in a workplace accident. The second witness — the helicopter pilot — died of a heart attack.