Lightning sparked the larger of the two wildfires that have scorched southern New Mexico, leaving at least two people dead, destroying 1,400 structures and ravaging more than 25,000 acres, the authorities said on Wednesday.

The blaze, known as the South Fork fire, began June 17 amid sweltering temperatures and was 87 percent contained on Wednesday evening, the Bureau of Indian Affairs said in a news release.

“The identification of the point of origin and all evidence and data support lightning as the cause of the fire,” the agency said in a statement. “Human activity and factors did not contribute to the cause.”

On June 23, the F.B.I. said that it was offering a reward of up to $10,000 for information leading to the arrest and conviction of the “person or persons responsible for starting” the South Fork fire and the Salt fire, the other major fire in New Mexico.

On Wednesday, the bureau said that the Salt fire, which the authorities said was 84 percent contained, remained under investigation.

The F.B.I. also said that it was still offering the reward for information “leading to the arrest and conviction of the person or persons responsible for starting the Salt fire.”

Both fires broke out on June 17, burning across the Mescalero Apache tribal area, on U.S. Forest Service land and in areas around Ruidoso. The fires forced thousands of people to temporarily evacuate the village of Ruidoso and surrounding areas.

According to the Western Fire Chiefs Association, the majority of wildfires in the U.S. are caused by people. Lightning is the most common natural cause, the organization said.